I was converting some sound files in iTunes, I don't really know the program that well, and came back into FCP and found all my sound was turned off. I had decible bars bouncing normally, but I had to re-boot my computer to get my sound back. What happened?
I occasionally get this issue as well. As you said rebooting solves this problem but it seems to be a FCP to OSX communication error. I don't know how I cause this to happen. Maybe the 10.3.3. update resolves this issue?
I had this problem for a while as well. If you go into the audio/video settings menu you can change your audio output from firewire device to built-in audio. That usually seemed to work for me.
