Most intercoms have an audio in and out. A real nice intercom (DRE is made one I've installed before) even mutes/turns down the music during radio reception if you like.
Most intercom installations, however, do not seem to wire up for the audio in or out. Some frown upon the idea of pilot listening to tunes. Captain's discretion....
I've got a cheepo flightcom portable intercom that worked great. Listened to tunes sometimes while flying jumpers, before talking to center (better than listening to jumpers!). The Audio Out was real nice for dual, especially instrument students-- could record entire lesson for student to review later (clearances, radio, instruction etc...). Think it cost me about $80. Audio in/out via sony mini stereo jack.
One last thing-- if you rig something up, suggest you test it out on FBO freq, or non busy ground etc. ATC might get testy if you bleed over music, especially if you goof up a clearance etc. Music in the backgroud of your transmissions on FAA tapes would look bad in LOI. If wired properly, this should not happen.
rock on.