"I've imported an audio file and it plays fine with the other realtracks.
But as soon as I move it to a blank "performance track", it doesn't play at all.."

You can just name any audio file my song.WAV or my song Soloist.WAV or my song Melody.WAV or my song Strings.WAV or AIF, M4A (in the same folder as the SGU)
then reopen the my song.SGU