This might be a silly question but is it possible to do software mixing with the square and triangle waves so you don't have to kill the music to play sound fx or does that only work with pcm?
You could do 4-bit DAC playback with the square channels, but that would need precise timing to write the new samples regularly, and you already have a nicer 7-bit channel to do the same with. The NES isn't really powerful enough to do software samples and run a game at the same time.
Of course, you could always fake it and use DMC samples that sound like square/triangle sound effects. Zelda 1's sword is a recording of the FDS sound effect played through the DMC channel.