If you're looking to really mute everything, as you're saying, you could do that with a command that triggers on keypress:
Mute system audio
Start VoiceAttack listening
Start Loop While : [{STATE_KEYSTATE:ALT}] Equals '1'
End Loop
Stop VoiceAttack listening
Unmute system audio
Where the "{STATE_KEYSTATE:}" token has the same key value as the command triggering key.
However, if you have a soundboard that would play in response to a spoken phrase, it too would be muted, as it's a system-wide setting.
You can mute individual applications, or specific devices(if your soundboard plays to a non-default one) to get around that.