Has anybody found occasions when one track prevents all the others from working? ie. If I mute that track, all the others play back normally. But if I unmute the track, there's a quick 'pop' sound and everything goes silent. Sometimes I seem to be able to coerce everything back into life by disabling plugins, but it's not always the same plugin, and these plugins work just fine in other songs.

After the "pop" sound - is the audio engine still on ?

Yes. When I re-mute that track, while still playing, all the other tracks come back in.

Strange. Sounds somehow like "Exclusive solo" going confused. Have you toggled Exclusive Solo (assuming you have SONAR 8 or 8.5)?

No, it's not exclusive solo. Something is going seriously wrong with the audio engine.