These commands were added to the game in 2016, I believe that was with toribash 5.0:
commands code:
/voicechat on|off - Turns the voicechat on or off.
/voicevolume [username] <0-1> -Changes the voicechat volume for everybody or a specific person.
/voicemute <username> -Mutes a player in voicechat.
/voiceunmute <username> -Unmutes a muted player in voicechat.
Voicechat volume is 0 by default, /voicechat on to enable
Default push-to-talk key is Q
It used to be a steam only feature, don't know if that changed, and I don't think it has been removed, it's just turned off by default and nobody uses it.