if you're looking for SNES (and not NES), the SNES 'sound' was that of very short loops for the samples. Since the SNES had only 64kbytes of RAM, all the game's instrument and sound effects had to fit in it. the data was compressed about 3.5:1, so that was the equivalent of about 224k. Figure half for sound effects and it means all your instruments had to fit into about 100k samples. So try making some short looping sounds. Most of my SNES instrument were only 5-10k samples long. If you make your music with instruments that fit into that small a space (your "instrument palette"), you'll get more of that "SNES" sound.
Also because of memory, sounds were sampled at low sampling rates, often as low as 8-10kHz (maybe for a trombone or kick drum), with maybe 22kHz being the high end.
Another aspect of SNES music was that notes didn't tend to overlap. What I mean by that is that normally if you limit yourself to 8-voice polyphony, you're not REALLY limiting it to 8 because when a "note-off' occurs, the sound still plays out it's decay, so some if the note is still playing into the next beat. So when doing SNES music, we'd very often shorten notes, which would ensure we didn't have unwanted overlap. So a full "quarter note" might be only 400 ticks (instead of a full 480).
Keep to those rules, and your music will probably be SNES-like in no time
In addition to using the bit-reduced samples, the chip is also able to generate simple white noise for sound FX. You actually heard that a lot in games - probably because using this feature saved valuable RAM space.
Also, the SNES' filter echo/reverberation might be noteworthy, especially Capcom made heavy use of this in most of their games. Nintendo's S-SMP works with finite impulse response, but you should be able to emulate the sound by tinkering with an algorithm reverb and listening to references.
Best thing you can do is compare your audio with the real thing and either try to get as close as possible or just get "that 16-bit feeling" with a more modern touch - nothing wrong with that!
