Project(s) mysteriously mutes itself

Hi, this has happened twice so far (on 2 STP projects).

In both cases I send SEQUENCES from FCP and everything is fine.

Now I adjust some levels on a clip, I press playback, no audio. And that's it, from then on soundtrack pro is unable to produce audio. The entire project is mute, I see the meters going up in the mixer, the monitoring is properly set, the mute project button is UNchecked, but even the STP loops are mute when I preview them in the loop browser!

In the second project, I didn't even change anything. Export from FCP, save, all is ok. I open the STP project later on, no audio. In both cases the only thing I could do was to re-export from FCP and redo everything I had done so far (fortunately, not much).

Is there anything else I could do to either prevent or fix this problem?

Thanks