Hold down the A/V button, that will turn off the Audio source, push it again to turn it back on.
My complaint about how the Z1/2 handle muting of the audio sources is that you can't differentiate different mute settings for different functions. For example:
1. During a phone call I want the music completely muted (would be great if it would actually PAUSE the source also, but I guess I'm the first person to think of that).
2. During route instructions I only want to attenuate the music.
It's impossible to do those two, as the same mute setting has to be applied to both, which means the music either stays on during a call, or stays at full volume during voice instructions. Because you can't set the level of instructions relative to the music volume (just an absolute level), if you have the music turned up fairly loud it can be really difficult to hear her voice.