There's a definite BUG on the expander that cuts out the sound on the Master, when it's applied to a track.
I tried it on the Demo, on vocals and bass separate, engage the expander and all sound drops out, either straight away or once there's a gap in the vocals, or if you stop/start it. CPU was cruising at 10%, no level in the output of the fx or master, still level movement on the other faders.
Check this thread for other comment, it seems to only do it on the demo, which is .ogg format.
http://www.cockos.com/forum/showthread.php?t=4993
Edit: Fixed in 1.71