Okay, so it's not OBS. Could still be Windoze or a driver trying to insist on being "schmart", and ending up stupid instead. Are you sure you've looked at *everything*? Some things are buried so that the id10t user that Micro$haft seems to insist on doesn't stub their toe on them.
Assuming HDMI, I doubt it. Picture and sound on that use the same wires.
It's based on the old analog TV standard, which had lots of dead time between lines and even more between frames, just to allow the analog circuitry to reset the line scanner and the frame scanner. And a sync pulse during that dead time ensured that the picture stayed in the right place on the screen. That's all still there on HDMI, except that the sync is now a digital code on the same wires that is not a valid color.
Like the captions on analog TV, sound on HDMI is also sent on the same wires during the dead time. So if the picture is good, so is the sound from a hardware perspective, and anything else that uses the same wires.
Even if it were a different format that sends its picture and sound on different wires, I have a hard time thinking of what else would cause that behavior. A new sound that starts perfect and then fades, then resets to perfect again when it stops and restarts, is almost certainly an intentional noise suppressor *somewhere*. It might not be intentional on *your* part, but someone wrote it to do that on purpose, and it's active somewhere in that signal path. Hardware failures are almost never synchronized to the content like that. Not at such a complex-logic level, anyway.