For each on-hold program, we’ll supply a unique HTTP url, which is a live mp3 stream of the program.  Within FreeSWITCH, this is handled by mod_shout. 

If you support playback of mp3 files elsewhere, mod_shout is probably already active in your configuration.

The recommended way to play such a stream url on hold in FreeSWITCH, is much like using a local_stream with a directory of audio files — the difference being that instead of audio files, you put a single file <name>.loc into the folder, and that file contains the url in a form like “shout://stream.cubemc.net/id”. 

In other words, it should be relatively straightforward to incorporate playback of the stream simply by dropping loc files into local_stream moh folders, without needing to modify dial plans.

The “mod_shout” page on the FreeSWITCH Confluence wiki describes the configuration for mod_shout.