@burnout426 IDK what in the world you just said but I will try if the problem re-occurs.

@burnout426 I don't know what you mean but I just deleted some random files (Opera GX Installers) and it works pretty well now. Will update if the problem re-occurs.

@burnout426 It started working again after I deleted some random unwanted files (one of the folders was termed Opera Installer).
sigh Some files sure are stored in weird, hidden places that I've never seen.

It seems that the problem occurs (I think) when I close a Private Browsing tab while Background Music is muted. I'm not sure if this is the exact case, but I'm speculating that this is the case.
With this information in mind, maybe you can deduct where the problem is sourced from?
Just for extra information here are all the extensions I have enabled:
Awesome Screenshot and Screen Recorder (allow in incognito)
BTRoblox - Making Roblox Better (both permissions off)
GoFullPage - Full Page Screen Capture (allow in incognito)
Google Translate (allow in incognito)
Opera Ad Blocker
Read Aloud: A Text to Speech Voice Reader (allow in incognito)
SaveFrom.net helper (allow in incognito)
Scribe — Documentation, SOPs & Screenshots (allow in incognito)
Search by Image (both permissions on)
Speedtest by Ookla (allow in incognito)

@dredgenneo How to fix the issue (temporarily):
Press Alt (or click the icon in the top-left) and click Update & Recovery...
Go to Recovery and click Recover... and then click Reset my data and Opera settings.

This is risky because you will have to bring back Everything from logins to extensions to site data, everything. So I recommend you Do Not at All Costs, take this temporary solution.

@burnout426

1. The whole private window
2. Yes.
3. Sidebar Visualizer Icon
4. Yes I know that but maybe it's because I'm closing a window with Background Music off.
5. No not really. Also there's no option to open Opera GX starting with Private Browsing Window (I think?) so basically no.

@burnout426 Problem is that even when I turn it off and turn it back on, the Background Music doesn't start again.

Edit: Also I disabled (muted) Background Music via the Sidebar Visualizer Icon. After some time that it is muted, I just close the whole Private Browsing window.

@burnout426 IDK what in the world you just said but I will try if the problem re-occurs.

@burnout426 Okay will try.

@burnout426 Okay for some stupid reason, I started up Opera GX, and somehow the Background Music was working.
I closed Opera GX and started it up again, and the Background Music just stopped working.

I did the weird things of starting up Private Browsing and disabling the Background Music (and Browser Sounds) from Easy Setup. and then closing the Private Browsing window. The normal window still had the options on, and the Sidebar Visualizer Icon showed a static image (I have Dynamic Audio Visualizer on). After turning off both options from Easy Setup and turning them on again, both of them didn't work (as usual). Usually the Sidebar Visualizer Icon just shows an empty box, to indicate that no audio is playing.

Another thing, I downloaded the Opera GX Installer again for a standalone installation. Thing is... there's no standalone installation option. When I went to "Install for:", it's just a greyed-out box with my device username written there.
So maybe there's no such standalone installation for Opera GX?

EDIT: Nevermind, I just deleted a folder named Opera GX Installer (from the time I downloaded Opera GX Installer again) or something like that. So maybe the conclusion is: Delete some related (and unwanted) files (like Opera GX Installers) and the problem will fix itself? I think this is tedious, as the searching for files in File Explorer takes a long time.

@burnout426 Problem is that even when I turn it off and turn it back on, the Background Music doesn't start again.

Edit: Also I disabled (muted) Background Music via the Sidebar Visualizer Icon. After some time that it is muted, I just close the whole Private Browsing window.

@burnout426 I have installed most of them from the Opera Add-Ons Website and for the ones that weren't in that website, I downloaded from Chrome Webstore.
I don't really know the specifications of from where I installed it.

I have tried that before but I'm not sure if I'm supposed to relaunch Opera GX after disabling an extension or not. If it is not needed then yeah, I have tried that and I didn't notice any difference.

Isn't Browser Sounds and Background Music exclusive to Opera GX? I don't think I will be able to see the problem in a standalone installation.

@burnout426 Maybe you can with the given extensions?

Awesome Screenshot and Screen Recorder (allow in incognito)
BTRoblox - Making Roblox Better (both permissions off)
GoFullPage - Full Page Screen Capture (allow in incognito)
Google Translate (allow in incognito)
Opera Ad Blocker
Read Aloud: A Text to Speech Voice Reader (allow in incognito)
SaveFrom.net helper (allow in incognito)
Scribe — Documentation, SOPs & Screenshots (allow in incognito)
Search by Image (both permissions on)
Speedtest by Ookla (allow in incognito)

@burnout426

1. The whole private window
2. Yes.
3. Sidebar Visualizer Icon
4. Yes I know that but maybe it's because I'm closing a window with Background Music off.
5. No not really. Also there's no option to open Opera GX starting with Private Browsing Window (I think?) so basically no.

@dredgenneo How to fix the issue (temporarily):
Press Alt (or click the icon in the top-left) and click Update & Recovery...
Go to Recovery and click Recover... and then click Reset my data and Opera settings.

This is risky because you will have to bring back Everything from logins to extensions to site data, everything. So I recommend you Do Not at All Costs, take this temporary solution.

It seems that the problem occurs (I think) when I close a Private Browsing tab while Background Music is muted. I'm not sure if this is the exact case, but I'm speculating that this is the case.
With this information in mind, maybe you can deduct where the problem is sourced from?
Just for extra information here are all the extensions I have enabled:
Awesome Screenshot and Screen Recorder (allow in incognito)
BTRoblox - Making Roblox Better (both permissions off)
GoFullPage - Full Page Screen Capture (allow in incognito)
Google Translate (allow in incognito)
Opera Ad Blocker
Read Aloud: A Text to Speech Voice Reader (allow in incognito)
SaveFrom.net helper (allow in incognito)
Scribe — Documentation, SOPs & Screenshots (allow in incognito)
Search by Image (both permissions on)
Speedtest by Ookla (allow in incognito)

@burnout426 It started working again after I deleted some random unwanted files (one of the folders was termed Opera Installer).
sigh Some files sure are stored in weird, hidden places that I've never seen.