Hey, I'm new to this site. I think it is great! Okay, here's the deal. I just downloaded Smule Ocarina. I was wondering how they made it so you can upload a song to the cloud. I might have an app idea that might incorporate this. How would I do this? What would I need?
Well you would have to define a format to play a song, for example MIDI, so you can Upload a MIDI file, ( not very heavy ) to a database (like mySQL) , and then you can download it and play it back with a synthesizer or something similar. There are some open source mobile synths that I think could be able to play MIDI, but you would have to check that out. Mobile Synth Open Source
I hope this helps you a bit, your question is very broad though, so there are lot of ways to accomplish your goal.