The N64 has no native sound processor -- just PCM sample output. Game developers often integrated some form of custom sequence or MIDI format, combined with PCM samples, and some playback code running on the N64 CPU. The miniusf file format is a bit of a hack, it basically packages a copy of the ROM together (sometimes stripped to just the audio code / data, but not often), a savestate of registers/RAM, and runs a full N64 emulator with the graphics code stripped out. [0]

So what you're hearing is just the playback code shipped in the game, plus all the samples it came with.

[0] http://hcs64.com/usf/