I think I have been having the same or similar issue. I upgraded my PC to Windows 11 a while ago, and I am using both latest SL official viewer and FS viewer (ATM). I can hear UI, environment, parcel music, voice but sound from internal browser and from prim/parcel media when the media is pointing to any web page generating sound, I hear no sound. The volume control is enabled on the media face or internal browser so I put that up all the way when I try to get it to work.
I was watching the Firestorm.log from user AppData Firestorm logs folder as I started to browse a youtube video... and I saw this odd line saying "NOT broadcasting negative" MediaOnAPrim ... LLVOVolume::mediaEvent
2023-01-10T07:04:43Z INFO # newview/llviewermedia.cpp(2585) LLViewerMediaImpl::navigateInternal : media id= d98f6ff4-3447-6f91-769a-c12c56744eb4 url=
2023-01-10T07:03:35Z INFO #MediaOnAPrim# newview/llvovolume.cpp(3177) LLVOVolume::mediaEvent : NOT broadcasting navigate (redirect)
2023-01-10T07:03:35Z INFO #MediaOnAPrim# newview/llvovolume.cpp(3177) LLVOVolume::mediaEvent : NOT broadcasting navigate (redirect)
2023-01-10T07:03:35Z WARNING #CoreHttp# llcorehttp/_httppolicy.cpp(434) LLCore::HttpPolicy::stageAfterCompletion : HTTP request 000002347689F820 failed after 5 retries. Reason: Timeout was reached (Easy_28)
...
2023-01-10T07:05:01Z INFO # newview/llviewermedia.cpp(2585) LLViewerMediaImpl::navigateInternal : media id= d98f6ff4-3447-6f91-769a-c12c56744eb4 url=https://youtu.be/jmKRgqWGrWc, mime_type=
2023-01-10T07:05:01Z INFO # llmessage/llcorehttputil.cpp(1186) LLCoreHttpUtil::HttpCoroutineAdapter::cancelSuspendedOperation : Canceling yielding request!
2023-01-10T07:05:01Z INFO # newview/llviewermedia.cpp(2585) LLViewerMediaImpl::navigateInternal : media id= d98f6ff4-3447-6f91-769a-c12c56744eb4 url=https://youtu.be/jmKRgqWGrWc, mime_type=
2023-01-10T07:05:01Z WARNING # newview/llviewermedia.cpp(2598) LLViewerMediaImpl::navigateInternal : MIME type probe already in progress -- bailing out.
I wonder this inconvenience caused by something in the viewer wrongly decided my volume control is set to negative volume? (I was standing 2m away from it, well within the sound fall off range setting - Firestorm-Releasex64 6.6.3.67470 - Otoa Kiyori (gyazo.com) )
I am probably looking at a wrong thing but I wonder if this helps the developer to find a solution to this probably rare problem.