There's no obvious 'mute output' control (there will be soon), but the easiest thing to do is to go to Edit->Controls, set Volume to Fixed, and lower the level to -40. It won't be totally silent, but pretty close. The other alternatives involve making a new instrument with silent samples, but that is more of a hack.

The next bugfix update (which really will come soon) will address this by providing a Disable Audio Output option in the Sound->Volume/iPod page, which will also reduce the CPU load by turning off effects, etc.

Thanks, looking forward to that update.