If your gateway is in a g.711 region (the default), the music should sound reasonably good. If your gateway is in a g.729 region (like across your wide area network), music will not sound very good. This is a known property of g.729; you trade off bandwidth for somewhat lesser quality voice and much lesser quality reproduction of music and other non-voice traffic.
This is also a possibility if you've set up Wideband MOH streaming. Wideband is pretty nifty, but we generally don't deploy it for our clients. The reason is that internal calls sound so good that there's a perceptible difference between internal and external calls where you have to shift down to g.711, and that of course starts the users complaining. If you just leave everything toll-quality, the users end up much happier overall.
If you're calling from a cell phone, keep in mind that the codecs they use are generally lower bandwidth (and thus quality) than g.711. Music on hold will never sound good on a cell phone.
