I have good news. I have found a workaround, and possibly also some information that can help fix the issue.
So, whenever I start the game with the microphone for my Bluetooth headset selected as default in the KDE Plasma sound settings, the audio recording stream for Void Crew gets created, so I can just change the default after launching the game to get mic input working.
So for whatever reason, when either Void Crew or Proton, I'm not sure which, looks for input devices it doesn't find the one set as default in Plasma sound settings. But setting another input device as default after launching the game works around the issue. It's still a bug, since just launching the game with previously working settings should be enough to have it work as before, but that's not happening and thus changing the default microphone after launching the game is a workaround for now.
Launch Void Crew.
Host or join a multiplayer game.
Press Esc, click "Microphone Muted" to toggle it.
Speak into your mic — icon does not light up.
Go to Settings > Audio.
Try changing Audio Input Device.
Apply and retest.
No mic input is recognized despite changes.
Set a different input device as default in KDE Plasma sound settings before launching the game, such as a Bluetooth headset mic. Then, switch back to the preferred input device after launching.
This forces Void Crew (or Proton) to detect and create the audio input stream. Once it's created, switching devices works as expected.
The game (or Proton) fails to detect the default microphone device during startup.
No fallback detection or runtime refresh of input devices after launch.
Suggests initial input device enumeration bug, likely related to how Unity/FMOD/ViVox interfaces with PulseAudio or PipeWire via Proton.
Nothing extracted yet.
Compatibility Report
System Information
steam-1063420.log
I confirm:
Symptoms
I can create and join online multiplayer games, but other people cannot hear my microphone input. KDE Plasma's volume mixer shows the game's output audio streams, but no input audio stream seem to be created for the game. Steam's own voice chat works fine, as does Discord. From what I've found out, the game uses Unity3D as game engine, Fmod as audio engine, and something called ViVox for multiplayer audio communication. Presumably the latter two come bundled with Unity3D.
Reproduction