I don't see this being very practical for home users for voipo. Sure it would be nice to hear music but that means voipo would need to look into royalty free music and have it available for clients to choose what to play in the vpanel or acquire a license or pay royalties for the music. Again, once the music is available for the clients, they are going to want to customize it and decide what song to play. The honest truth is that VOPO is as low as $$8.25 for 5000 minutes and I would rather the company focus on keeping prices low and the service constantly working.
Now if you have a business service through voipo, can't you use some kind of hardware to connect to the phone and/or ATA to play the music? Don't let the RIAA hear you play your favorite lady gaga album on hold since you probably did not purchase the rights to stream that album.

If you really want music on hold, look into an open source pbx system.