Hold 'Space' to talk then release to mute in meeting just like a walkie talkie (solve forgot to mute / unmute mic.)
This can prevent people speak then found forget to unmute microphone, also can reduce echo.
spacebar would be *very* inconvenient given it's frequent usage outside the scope of the voice-chat, it would be one of, if not the *worst* choice.
The user needs to be able to set the keyboard (or input in general) command they personally choose as all of us have our own variances of duties.
by default the ` key would be ideal since it's very rarely used.
others may opt for a right hand controlled key such as \
but programmers and other IT folk would probably find this annoying...
others still may prefer something such as the numpad + key, since it's within reach of the right thumb while mousing
or even better, those with additional mouse buttons could utilize those as a hotkey for Push-to-talk