Implement a MIDI 1.0 sequencer and get it to play back SMF files with a sine wave synth - you can have it output a WAV file, or learn an API to do realtime rendering. It's not a large spec, there are lots of old documents on how the protocol functions in practice, and then once you start getting it working you'll get results instantly(lots of SMF files around to test with) but will want more features, better synthesis; complications start to arise and you will then pick up a lot of knowledge by doing.