I use the Fisher-Price Rainforest Peek-A-Boo Leaves Musical Mobile ($30 on Amazon as of this posting) for both my kids. The music is quiet and the motion fairly slow and gentle. I had my daughter in my room for the first 9mons (and now my son) and I wasn't bothered by the mobile at all. Most the time I could hardly hear it. My daughter still uses her's every nap and bedtime. My son is just discovering it but seems to enjoy it.
The only downfall (to me) is that the motors seem to wear out after a year or so and the music plays but it won't spin. Luckily, Fisher-price is pretty good about sending replacements.