All it's doing is converting the audio to 4bit depth with a sample rate of 11khz.
This can be played back by the gameboy, but:
It will sound like shit.
It will be too large to use a whole song.
It will probably be pretty processor intensive to play back a long file.
You aren't converting it to use in programming, you are converting it into a format the game boy can deal with.
If you want to write music to use in a game, maybe this is a better place to start.
http://jiggawatt.org/muzak/xpmck/