Tricky one. I wrote a soundtracker (VSD Tracker) in DBPro when it was very young, it had support for pitch, volume and panning wave samples and that's about it. With AppGameKit, it is possible to pan a sound and adjust it's volume - however I don't think it's possible to change the pitch, which kinda negates it as a music app language unfortunately. Maybe this will be addressed at some point, but it's not vital so I think you might be in for a long wait. Exporting the music to a MP3 or even just a wave file isn't possible yet, not unless you want to get your hands dirty
Really, a music program has to have deeper access to sound hardware than AppGameKit or even DBPro affords us. They tend to use custom sound libraries that put the sound output on a controllable or higher priority, it needs to access, monitor, or modify the sound data in it's raw form - so AppGameKit is probably a stretch.
So... AppGameKit probably won't support the features you need in a music app unless it was a really straightforward music app, or didn't allow actual music editing - like an eJay program for example. It won't be able to easily export the data.
The biggest issue though, is competing in this market. For example, Garage Band on the iPad is an astronomically impressive tool, it has features that no small team, let alone a solo developer could hope to compete with. In fact, the only mobile music app that could compete with it, is Fruity Loops Mobile, which doesn't even exist (unfortunately).
I would suggest looking at DBPro if you really want to make a music app easily - look at the sound plugins and what they can offer. It would be PC only, but something that runs on a PC laptop is actually pretty decent option these days.
I got a fever, and the only prescription, is more memes.
