A song dumper/inserter for Metroid.
There are four channels for sound. Two square-wave channels can have a relatively simple “instrument”. You can't set the volume for individual notes, or use more than one instrument on a channel. Then there's the triangle-wave channel, which is a flat tone at a fixed volume, that most games use for bass. There's also a single percussion sound that's used in the Brinstar music (there are a couple other percussion sounds that aren't used, but they seem mostly useless).