In our office, we ended up with a rather unique music on hold solution. A CD player and so on can be difficult to rely on. Instead, I set up an old (pentium 2-400) computer as an MPD server.
MPD (Music Player Daemon -
https://www.musicpd.org/) is a program that you can run in the background on a linux computer. It doesn't use much in the way of resources, and in the case of this comp, I installed the latest server version of ubuntu, along with the standard mpd packages. You can control the mpd program from other computers on the same network, using any number of client programs, such as the Gnome Music Player Client.
Other solutions might be quicker, but it was a fun little project for me, and it was essentially no cost. In this case, I can play ogg, mp3, and wmv files, among others. The important part is to make sure that you have open licensed music, such as the music available on the creative commons website, or the WikiCommons.
Bryan