Is this an APX you're referring to? The APX CPS says a lot more about Voice Mute than that.
Basically, it's intended to mute the RX audio from a channel until a call or page is received. It works in conjunction with In-Call User Alert.
It was never intended to be used as a general mute feature, but again...if you spend some time learning the features in CPS, you might be able to use Voice Mute as a hack to mute all RX audio. It's possible, however, that as soon as you key up on that radio Voice Mute will be disabled automatically...I don't recall, you'll have to experiment with it yourself.