Carla Schroder tells you how to configure Asterisk with hold music. Via VoIPPlanet: VoIPowering Your Office with Asterisk: Soothing the Savages with Hold Music.
One of the more fun aspects of running an Asterisk server is choosing your own hold music. Hopefully your callers are not sitting on hold for long periods of time, but as long as they are it's nice to give them something pleasant to listen to. It can be music, inspirational speeches, comedy routines—even Internet radio.