The NES is locked at 30 FPS on the sound channels. I can only buffer sound in between screen refreshes, so i'm stuck there.
The hardware doesn't really allow for sweeps or fades. Fades you can kind of fake by manually playing the note at a progressively lower volume, but sweeps can't really be faked. You can pull something similar off by layering the sound channels, but i haven't really tried. The NES doesn't let you play a second note without a rest in the middle, that's why NES music is always so staccato.