I am experiencing the same issue with a WebRTC web app I have developed. It uses audio and the incoming audio is about 2/3 of the volume that it used to be with iOS 14.8 and below all the way to iOS 11. This can also be demonstrated by going to:
https://webrtc.github.io/samples/src/content/getusermedia/audio/
or
https://appr.tc/
And testing iOS 14.8 and beta tester release 5.1Beta3 side by side. I have verified this on both iPhone 7 and iPhone XS
The outgoing microphone level is normal. The problem is with the playback audio.
marcusgilson
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We just re-tested with the newly released 15.1 and the issue is still there with low audio with a WebRTC audio stream.
Tested on an iPhone XS. Our users will need to refrain from installing 15.x until this is resolved.