In April 2019 when I had the problem, I was on:
Windows 10 Edu (1803) 64-Bit
Opera 58.0.3135.127
WidevineCDM 4.10.1196.0

Now in November 2020, I'm on:
Windows 10 Edu (1909) 64-Bit (unable to receive 2004 and 20H2 updates yet)
Opera 72.0.3815.207
WidevineCDM 4.10.1610.0

Some more that show the problem for me.

https://twitter.com/thesimplecrew/status/1117105076584108034
https://twitter.com/kgmpike/status/1117221328468238336
https://twitter.com/bitandbang/status/1117225129539506177
https://twitter.com/alyssarose1391/status/1117303511018983425
https://twitter.com/sarancr7/status/1117286932742987776
https://twitter.com/KayeMaxilom03/status/1117296958081658880

The audio cuts out at 9 seconds (for all of them) and then comes back for the last 2 seconds (on some of them).
Only the last one worked for me without dropping audio, but the audio did jump at 9 seconds for the remaining 2 seconds of the video as the length of it was 11 seconds in total.
I guess if the video was 12 seconds long instead of 11, then the 10th second would have dropped out? This seems to be the pattern.
Also all of these are mobile videos again (not sure of the operating system but I'll guess it's iOS if they're using TikTok + their age). The operating system is the next thing to find out here and also if any landscape mobile videos have this problem too (still looking for computer uploaded videos too).

Edit: At the time of writing this Twitter is flooded with videos of the Notre Dame cathedral fire. All landscape mobile videos seem to be working fine, but some vertical videos aren't. I'm not sure if it's because people are downloading them and reposting them (so Twitter reprocesses the video's audio) or if it's because of the OS the video came from.

https://twitter.com/thesimplecrew/status/1117105076584108034
https://twitter.com/kgmpike/status/1117221328468238336
https://twitter.com/bitandbang/status/1117225129539506177
https://twitter.com/alyssarose1391/status/1117303511018983425
https://twitter.com/sarancr7/status/1117286932742987776
https://twitter.com/KayeMaxilom03/status/1117296958081658880

If you wanna compare two here is a link.

Length : 56 seconds,audio is gone after 9 seconds
https://twitter.com/erdalbiroll/status/1116473516684132352

Length : 44 seconds, audio is perfectly fine
https://twitter.com/Red_ZaZa_/status/1116575919215276033

The first video is screencaptured on a mobile (iPhone), also horizontal.
The second video is landscape (manually pillarbox'd), possibly from a computer.

It seems the theory that the problem is with mobile recorded/processed/uploaded videos is looking true.

I'm curious to see if we can come across some landscape/horizontal videos with the audio problem.

Length : 56 seconds,audio is gone after 9 seconds
https://twitter.com/erdalbiroll/status/1116473516684132352

Yes, thanks. That's another one.

Length : 56 seconds,audio is gone after 9 seconds
https://twitter.com/erdalbiroll/status/1116473516684132352

Length : 44 seconds, audio is perfectly fine
https://twitter.com/Red_ZaZa_/status/1116575919215276033

Edit: Also the fullscreen solution works.When the video is fullscreen, audio spikes up at 9 second so it is either a problem about how opera handles .ts file extension or how Twitter streams these.If you go fullscreen before 9 seconds you can turn it back to normal and it works fine.

For me, the audio in the links:

Doesn't work
Works
Deleted
Works
Works
Works
Works
Works
Works

Interesting that you get different results.

As for your settings in Opera and Windows, I don't think they matter.

@luca786toni said in Twitter video's sound gets muted.:

The only solution I've read here so far is making the video full screen to hear all the audio.

With https://twitter.com/ShyCountryCutie/status/1104993101028896768 for example, switching to full screen does make it play. However, it never gets louder at 9 seconds (in Chrome either) like the non-full-screen version. Twitter probably sends the video in a different way which doesn't present the problem.

@luca786toni said in Twitter video's sound gets muted.:

They're all from mobile and they all look like reuploads

Yeah, not sure. I did ask a few Twitter users if the increased loudness in Chrome at 9 seconds is intentional or not. Still waiting for replies. It could be Twitter is boogering them up.

Confirmed. And, here are some more (if you find any more, please post links):

https://twitter.com/xxGodsLastGift/status/1114036356576088064
https://twitter.com/xxGodsLastGift/status/1111876353094238208
https://twitter.com/ShyCountryCutie/status/1107543612089200642
https://twitter.com/ShyCountryCutie/status/1106975279501602816
https://twitter.com/ShyCountryCutie/status/1106716407826284544
https://twitter.com/ShyCountryCutie/status/1104993101028896768
https://twitter.com/maeve_mcdermott/status/1113822509940531200
https://twitter.com/officerdaddo/status/1110918526234230785
https://twitter.com/officerdaddo/status/1097321507766919170

It's a known issue (DNA-74703) and why the audio cuts out is known.

For me, the audio in the links:

1. Doesn't work
2. Works
3. Deleted
4. Works
5. Works
6. Works
7. Works
8. Works
9. Works

The ones that work all worked immediately first time. They're all from mobile and they all look like reuploads - as in they were downloaded from the original poster's account rather directly uploaded from the app that recorded it. This could contribute to more compression of the video files and also stripping out multiple audio channels down to just 2 channels (would have perhaps even forcing 2 channels only (or even mono channel) thus fixing any issues - I say this because they worked for me for some reason).

Though, unlike the Bugzilla report on Firefox for this problem, this all sounds like it is more than just a multiple channel problem in Opera. I'm not even sure multiple channels are the issue for Opera.

I'm really curious to see if this problem is present in videos not recorded/edited/encoded/uploaded from mobile, like a trailer or a meme uploaded from a computer to Twitter or wherever.

Here are the only flags I have toggled (not that they work properly anyway, overlay scrollbar is still broken - also just noticed that taking screenshots of the flags page cuts the image in half... we're just stumbling across bugs left and right at this point ):

Here are my Windows Performance Options (which also should have no affect):

_

_
Here are my Opera settings that have worked for some when changed (mostly Chrome users?), for whatever reason (makes no difference to me, nor even sense why it would change anything) (my ad blocker is turned on):

Here are my 3 Opera extension that are enabled (extensions page also does not work well with screenshot tool):

The only solution I've read here so far is making the video full screen to hear all the audio. Not an ideal permanent solution.

Here's another example

Confirmed. And, here are some more (if you find any more, please post links):

https://twitter.com/xxGodsLastGift/status/1114036356576088064
https://twitter.com/xxGodsLastGift/status/1111876353094238208
https://twitter.com/ShyCountryCutie/status/1107543612089200642
https://twitter.com/ShyCountryCutie/status/1106975279501602816
https://twitter.com/ShyCountryCutie/status/1106716407826284544
https://twitter.com/ShyCountryCutie/status/1104993101028896768
https://twitter.com/maeve_mcdermott/status/1113822509940531200
https://twitter.com/officerdaddo/status/1110918526234230785
https://twitter.com/officerdaddo/status/1097321507766919170

It's a known issue (DNA-74703) and why the audio cuts out is known. However, it's not an easy fix to work around the issue. It needs reworking of the audio handling to fix it. Same for Vivaldi. Same for Firefox on Mac (I hear). The audio works fine on Firefox for Windows as they did some reworking that makes things work. As for Safari, the audio works, but Twitter serves up the audio to Safari in a different way, so that's not a great comparison. The reason it works in Chrome is because it uses ffmpeg for media instead of the Windows Media Foundation. It works in Edge too.

So, it's possible to fix in Opera. It's just going to take some work if Opera is able to provide the dev resources to do it.

It shouldn't make a difference, but try it anyway. In Opera, goto the URL opera://flags/#autoplay-policy and set it to "No user gesture required".

Tried it, still the same.

Here's another example that I bookmarked:
https://twitter.com/juventusfcen/status/1114541160213749760

Different length (1:23) yet the audio still cuts out at 9 seconds and comes back for the last 2 seconds (on the second playback).

I've noticed that it's from videos recorded & uploaded from mobile (vertical footage). Maybe they recorded it with the stock camera and uploaded it to Twitter or used the in-app camera (doubt it)?

If it is because of mobile (where the footage was recorded) then I don't know if it's the iOS's encoder or Android's encoder that Opera doesn't play well with.

The more patterns we can pull from more examples that we post, we can probably narrow down the cause.

Audio cuts off 9 seconds in. The last 2 seconds can be heard after a second playback: https://twitter.com/xxGodsLastGift/status/1114036356576088064

I can confirm with this one. This is a decoding issue. There's something significant about 9 seconds. If you listen to it in Chrome, at 9 seconds is when the sound switches to louder etc. and that's exactly when the audio cuts out in Opera.

Looking at opera://media-internals after playing the video, I see it's streaming blobs of the video/audio data and it switches to mono or something midstream.

Selected WMFAudioDecoder for audio decoding, config: codec: aac, bytes_per_channel: 2, channel_layout: 3, channels: 2, samples_per_second: 44100, sample_format: 2, bytes_per_frame: 4, seek_preroll: 0us, codec_delay: 0, has extra data: true, encryption scheme: Unencrypted, discard decoder delay: false

Selected WMFAudioDecoder for audio decoding, config: codec: aac, bytes_per_channel: 2, channel_layout: 3, channels: 2, samples_per_second: 44100, sample_format: 2, bytes_per_frame: 4, seek_preroll: 0us, codec_delay: 0, has extra data: true, encryption scheme: Unencrypted, discard decoder delay: false

audio decoder config changed midstream, new config: codec: aac, bytes_per_channel: 2, channel_layout: 2, channels: 1, samples_per_second: 44100, sample_format: 2, bytes_per_frame: 2, seek_preroll: 0us, codec_delay: 0, has extra data: true, encryption scheme: Unencrypted, discard decoder delay: false

Selected WMFAudioDecoder for audio decoding, config: codec: aac, bytes_per_channel: 2, channel_layout: 3, channels: 2, samples_per_second: 44100, sample_format: 2, bytes_per_frame: 4, seek_preroll: 0us, codec_delay: 0, has extra data: true, encryption scheme: Unencrypted, discard decoder delay: false

Same issue with Vivaldi. I guess the Windows Media Foundation that Opera and Vivaldi use to decode the audio doesn't like the channel count switching like that.

Using version 58.0.3135.127
Win 7 64bit

Windows 10 Edu (1803) 64-Bit
Opera 58.0.3135.127 (latest as of writing)
WidevineCDM 4.10.1196.0

Audio cuts off 9 seconds in. The last 2 seconds can be heard after a second playback: https://twitter.com/xxGodsLastGift/status/1114036356576088064

Tried without ad-blocker
Tried in Opera Beta 60.0.3255.20 (no extensions) (latest as of writing)
Tried in Opera Developer 61.0.3275.0 (no extensions) (latest as of writing)
No flags touched
Tried an new user account, clean profile, same problem

Works in Chrome 73.0.3683.103, Firefox 66.0.2, and Edge 42.17134.1.0 (all latest as of writing)
On these browsers where it works, at 9 seconds the audio jumps in volume and also has a different sound to it as if it changed mics(?)

I tried replacing the files in Opera's WidevineCDM folder (C:\Users[username]\AppData\Roaming\Opera Software\Opera Stable\WidevineCDM\4.10.1196.0) with the files from Chrome's WidevineCDM folder (C:\Program Files (x86)\Google\Chrome\Application\73.0.3683.103\WidevineCdm). Even tried replacing the whole WidevineCDM folder with Chrome's.
Still same problem.

Opera is using WidevineCDM 4.10.1196.0 while Chrome is using WidevineCDM 4.10.1303.2

This is intermittent. Some videos work fine, others don't. The video in the Tweet that I linked above fails every time in Opera and works every time in every other browser.
Twitter (and Facebook?) is almost unusable in Opera with this problem.

There's another thread on this too: https://forums.opera.com/topic/30327/twitter-videos-sound-problem