If your metronome in MainStage is turning off sound for a patch or the Tonic Pad, this is because you haven't turned on MainStage's clock yet (the green play button right next to the metronome on the top right). Anytime you start MainStage's clock all midi and audio is reset.
You need to do things in this order:
- Turn on MainStage's clock (green play button right next to metronome)
- Turn on Tonic or play patch
- Turn on/off metronome.