J.K.
We like this one a lot: http://www.toysrus.com/product/index.jsp?productId=3132749. My son is nearly 3, but it could certainly be enjoyed at any age (he's had it for more than a year now). We bought it to use in the car, but it works well in the crib, too. There is a strap in the back so that it can be attached. It plays music (two volume settings) and lights up with the beat as it plays. It doesn't play for long on its own... maybe 3-4 minutes, but it's very soothing music. We like it...
And yes, Discovery Toys are a great place to look, too!
Best of luck to you,
J.