@Picketfence There doesn't seem to be a lot of information available about this set up. My GUESS is that because local/couch/co-op play isn't available yet for anything (except Ghost Recon Breakpoint which uses the Stream Connect feature) that only one connected controller is going to be recognized for sound.
The claim by Google is that eventually there is going to be no-limit local couch/co-op play depending on the game you're playing:
"How many players can play on a Stadia game?
The number is dependent on the game you're playing. Stadia will support full multiplayer titles, as well as couch based co-op and single player titles."
https://support.google.com/stadia/answer/9338946?hl=en
So probably once this feature is fully implemented you'll be able to get sound on all connected controllers for games with local couch/co-op multiplayer. But until then, probably only one controller is going to receive sound.
Like I said this is JUST a guess based on what I could find. I hope it helps.
Yeah Problem still exist on any Game no matter if I play "Couch-Coop" in Front of my PC or Chromecastz Ultra. If 2 Players are Connected locally no matter if Sound should come trough Controller or one Player Connected to PC/Chromecasting TV and one via Contreoller only one Player is able to have Sound.
So the only way to play Couch-Coop is when we share the same Soundsource(eg TV Speaker aso).