You blew your speakers listening to AC\DC?
It's happened to me. Twice.
Seriously though, does the headunit appear to go through the whole track with just silence? Same result with other tracks and playlists?
Some possibilities:
1. Some headunits have separate memories for each source. You might have something set wrong for the external CD source on your headunit.
2. Some files could have been corrupted causing the phatbox to act strangely.
3. The cable could be loose.
4. If I remember, there was another issue similar to this on the official forums once and it turned out to be a bad Phatbox.