What version of Windows? (the "OS" value as seen at the URL
Windows 10 OS Version 1909 (Build 18363.720)
Using 64-bit Opera or 32?
What sound card/chip do you have and what driver version?
Not really relevant - the Windows sound system handles that, and is currently set to four channels. The current default system is an E-MU 1616m
Do you have more than one sound device?
Yes, but irrelevant
Is your sound device set as the default?
One is, of course.
Do things also work fine in Vivald, Brave, Chrome, Chromium, and Chromium-based Edge?
Surround sound works as it should in Firefox, Chrome and Edge. It used to work in Opera but not now. All other programs (not only browsers) also work as they should.
Things still broken in Opera Beta and Opera Developer from https://www.opera.com/download?
Same in Beta and Developer
When you say, "Surround audio doesn't play", are you saying you only hear audio out of 2 of the speakers while other browsers produce audio out of all 4?
No sound; nothing plays.
I note that on a page displaying default HTML5 audio players, those linked to surround files have the play button greyed out.
Do you have an example link?
After trying to play the video/audio, if you goto the URL
opera://media-internalsand expand the section for the player in question, it should tell you what type of audio it's trying ot play and what errors there are.
The player in question is not even listed.