Hrm. The description says it is only 3.2 inches high! Looking at the picture, I can't see it actually being that short, but maybe it is. I looked up "baby mobile" on Amazon and there were a couple for 10 bucks. Maybe you bite the bullet and get a cheap mobile and then attach yours to the clamp instead? Most of the standard baby mobiles have the ring where you attach the music box, and then a hook on the music box to attach the actual mobile, so that should give you more play in the height. If you wanted you could attach the mobile directly to the ring (using a keyring or something) instead of the hook on the music box.
I think for safety purposes, the mobile should be pretty high up. They make mobiles that are interactive for baby to kick and bat around, but I just don't feel safe about string hanging where baby can reach. I think when baby can stand up and reach the mobile it's time to take it down.