I've not had the "no audio" symptom in a while, but I always just pulled the Sync fuse for about 10 seconds. I'll have to try that power off/on method if it should happen again.
Ineresting about MP3gain. I ran all my MP3s through it as well, but when played through Sync, the volume level does sometimes varies from song to song. I read that MP3gain uses the APEv2 tag information and only works with devices that know how to read the tag. MP3gain does not actually alter the MP3 data itself.
I've never got a confirmation (although I have asked) but maybe Mike could confirm if Sync does acknowledge and process the APEv2 tag.