I came to this conclusion too, worked perfectly before but today no streaming audio. Left the Beta and streaming audio has returned. However performance while streaming in Stable is now very stuttery especially the audio so now I've got to figure out what settings I've messed up to cause that...

Streaming Spiderman Remastered was a stuttery, crashy, glitchy mess. After even more hair pulling I resolved it, I think it was this: changed the client (deck) streaming resolution to automatic (1280x800) instead of a higher res I had set for other reasons a long time ago. This fixed the constant stuttering and crashed streaming issues I was having. I also changed on the host PC: disabled "change desktop resolution to match streaming client". That may have been causing some issues as well.

I have the same issue using the Steam Link, no audio is being streamed.

Can confirm. no audio via steamlink

Also can confirm I ran into the issue, certain programs that typically only play with "All System Audio" set to record was completely muted. Switched back to stable build, audio was restored....

Curiously, for certain games / programs that worked fine with "Game Audio Only" they still worked, I think it's something specifically with that option. (Or it could just be me)

Bumpy bumpy, confirmed, rolling back to Stable fixes this so it's a Beta issue

Can confirm!

In beta:
Record audio from... :
Game Audio only - Captures no sound
All system Audio - Captures no sound
Game and Selected Programs - Captures audio only from selected programs

This happened around 16-17th October (recordings on 16th were fine, but on 17th the audio started missing)

Rolling back to stable fixed the issue.

Came here to report the same. Audio device refuses to switch on this beta version. Picture shows up on the TV (thru steam link) just fine, but audio absolutely refuses to budge from the typical output I use when at my PC