GarageBand, iTunes: GarageBand audio not audible in movie shared to iTunes.
GarageBand, iTunes 10 for Mac
If you send a movie to iTunes via the Share menu in GarageBand using the Full Quality video setting, the audio you added in GarageBand may not play in iTunes. You may hear the movie's original audio track instead.
When using the Full Quality video setting, GarageBand exports your audio to a track and preserves the movie's original audio track. iTunes defaults to playing the first track in the Audio and Subtitles list, which may not be what you expected.
To resolve this issue, use a different video setting instead of Full Quality when exporting from GarageBand. Alternatively, you can choose Controls > Audio & Subtitles and choose another audio track.