Sorry for the bump... I just noticed how old this was and already wrote this

anyway...
Very, very cool!!!
I'm glad you included the midi as well. For people who aren't as musically inclined, maybe you can add an option to set the key as well?
It would also be cool to save the channels individually, for re-use in other tracks. Like finding a cool violin pattern and saving it for re-use, which gets transposed and stretch to the new tempo in another track. This would allow users to have some motifs in their game throughout separate tracks!
I noticed the loading screen takes quite a while when generating a new song, even though it says it will take a few seconds. Maybe add some flavor loading text (adding melodies, spicing up riffs, generating song, mixing volumes)? I found myself getting bored and going through other tabs while waiting for it to generate.
Also, 240 credits feels like an odd number to calculate when paying for a song. I get 3.3 songs for $5? Kinda confusing.
On a wide screen 1440p monitor the screen also stretches from end to end which can feel claustrophobic.
All in all pretty cool