I believe you’re wanting to do something similar I tried to setup when I first moved to Asterisk:
Internal dial a music stream - assigning extensions to streaming music-on-hold for internal calls - FreePBX / Configuration - FreePBX Community Forums
In short, you can use something that will multicast the MP3 and have your phones setup to listen and play that multicast. It works, but it is the same as a multicast page, it will continue to play unless you cancel it, or you have the phone set to allow barge paging or calls.
Chris