Stuart, in my case the muting occurs more often in standalone than in my DAWs, but in my DAWs it's more muting in Ableton Live than in Cubase (7.5) or Studio One. Sometimes i just have to switch patches to get sound back again, sometimes i have to completely reload the plugin or restart the standalone.
Sometimes i can work for 20 minutes without problems, sometimes only for 5 minutes or so. It seems that the muting occurs only in a longer interval when i'm not playing or doing something in my DAWs.
PS: It's not a MIDI issue, i can see the GUI keyboard's keys working when sound is muted.