After Effects: Audio Not Playing - in AE CS4
Maybe a dumb question, but is the loudspeaker icon enabled next to the music composition layer, when you place it in your main comp?

Also, sometimes if I have weird things happening with mp3 files, I convert them to 44.1khz 16bit WAV files instead, just to rule out any mp3 oddness...

Have you definitely connected the Audio Spectrum effect to your audio layer, by choosing it in the Audio Spectrum's "Audio Layer" effect setting?