Music on hold comes from a MOH source and is plugged into the cabinet on the MOH port :-)
Your ACS module has an RCA jack for the music input. You get a source, like a music on hold player
Or, you can just plug in a CD player or radio. The Intellitouch unit comes with a program called Message Studio and lets you record your own messages or mix your own music (they also provide royalty free tracks) to generic "Thank you for holding" messages.
There is an Australian company that sells you a mini production studion that runs on a PC. It's a bit of an overkill for the average user, but we still do that for messages that need to be scheduled.
Anyway, a long winded answer but I hope this helps.