With the release of FreePBX 2.5, Music On Hold comes with the new feature "Add Streaming Catagory." This addition to the Music On Hold module allows the Integrator to offer MOH streamed from the Internet or via the Line IN port on a sound card as a catagory.
In this example a Police Scanner is connected to a Line In port on a FreePBX 2.5.0 / Asterisk 1.4.21.2 system. The directions used to get Audio in and available are located here - http://www.sthomas.net/go/blog/view/38 NOTE: Skip editing musiconhold_X.conf. Use the GUI.
This example uses a scirpt created in /usr/sbin/ called "ast-playlinein" and it is used to control the parameters of "arecord" which is the program doing the heavy lifting here. Pointing at the script was all that was needed.
Streaming from the Internet is not that different. A good link to get started is http://www.voip-info.org/wiki-Asterisk+config+musiconhold.conf