Here are two music-related tools available from Apple that should be of interest to exhibit developers. I could imagine either program being fun to noodle around with in a Music Gallery or a Multi-Media Lab.
The first tool, Logic, is actually a tool that's been around since Atari days, and has recently been acquired by Apple. Some people have described Logic (or its current incarnation, Logic Express 7) as "Photoshop for music." I suppose that's because there is a strong visual underpinning to how Logic works. In any event it seems like a great tool to let museum visitors play around with.
The other music tool (coincidentally with a strong visual component as well) is the new part of Apple's GarageBand program called "Magic GarageBand." Magic Garage Band lets you move visual representations of musical instruments on or off a "stage" to build up (and then record and/or transcribe) musical compositions. It's just been released, so I've only just started fooling around with Magic GarageBand, but it seems to hold great potential.