That music engine sounds unfinished. The square wave channels don't change pulse width. The percussion doesn't even have the hi-hats on them. (An easy way to do hi-hats is to add a 1-frame burst of high-pitch noise before a kick or snare note.) In all, it reminds me of Hatris for NES or perhaps a couple of Spectrum Holobyte's Game Boy music engines. Either that, or some Color Dreams or Sachen game.

I notice that the NSF is under 11 KB unzipped. Would you guess it was impossible to do any better in that amount of space?