The SUNSOFT 5B, which was a modified version of the Sunsoft FME-7, contained the Yamaha 2149 PSG, which is functionally identical to the General Instruments AY-3-8910, which appeared in the MSX, several arcade cabinets, and of all things, the Intellivision. It's generally felt that the AY and YM slot in-between the SN76489s and POKEYs of the world on the one hand, and the 2A03s and SIDs of the world on the other...in other words, it's generally held to be an inferior sound chip to the one in the NES.
But what Gimmick proves for us is that even though it's inferior, the inferior, paired with the superior produces something FAR better than what the superior can produce all by itself.
A lot of people, I think, assume that the FME-7 added the sampled bass channel, but no, this is just stock 2A03. NES has a 1-bit PCM sampler that does a really crappy job of sampling...but can do it - without tying up any other resources in the sound chip. You will occasionally hear it produce percussion (think Super C, or Super Mario Bros 3), or it will be used for short voice clips, or for sound effects, but it was VERY rare to hear it used "musically". As far as I know, only Gimmick, Batman: Return of the Joker, and Fester's Quest use it for bass guitar (though the bass is so poor that it's almost unrecognizable as a bass guitar sample, or anything that began its life organically, for that matter)
What the YM adds is three more square wave channels (they can do 25% and 85% duty pulse waves too, but I do not think they're used that way in this soundtrack). Big deal, right? Well hold on. Not only does that bring the total number of channels to eight (same as SNES), but it completely frees the 2A03 up to just use asymmetrical pulse waves on those channels, and not have to worry about plain square, while still keeping those wonderful square tones in the mix - moreover, the square waves on the AY/YM are a little "rounder" and more "haunting" sounding than the purer, but blander squares of the 2A03. So, these three extra square waves turn out to be a huge boon after all!
The typical sound scheme of this soundtrack is 2A03 PCM bass (with the PCM also pitching in a little sampled percussion in-between bass notes), the white noise handling the lion's share of the percussion, with the triangle wave - usually used for bass freed up to mostly devote itself to supporting the percussion with portamento falls "tom fill" drums. The 2A03 pulse wave channels, not needing to produce plain squares can just stay in 12.5%, 25%, and 75% pulse wave mode for tonal diversity and inflection. And then, as I said before the YM brings a trio of their haunting, phasey, wonderful plain square waves.
In fact, the soundtrack doesn't deviate from this scheme til about 2/3rds of the way through, when "Just Friends" starts at 19:23. This track features a square wave bass (provided by the YM), subtle sampled percussion, YM rhythm squares in the melody and harmony register, NES pulse waves, and if I'm not mistaken, the triangle wave is buried up in the squares (it might also just be absent).
Next up, at 21:11, is my favorite track of the whole game, "Sophia". It starts with NES sampled percussion, and a YM bass line with a pair of square waves (to get the fatness, phasey-ness, and warmth) until the bridge at 21:37, when it drops to just a single square wave, since that one is needed elsewhere. And then, at 21:49, the chorus comes, which features NES sampled bass once again. The chorus also features a double melody of NES triangle wave, and NES pulse wave. Even cooler is the plain triangle wave solo at the very beginning of the track at 21:17. Keep in mind a few things: the triangle wave has a really smooth sound, that kinda meets square and sine in the middle, if not even leaning closer to sine (though it has overtones unique to it that neither sine nor square have), and that it's normally used for bass or drums, so you don't often get to hear its soothing call in the upper register.
Paradox, and Innocent go back to the Gimmick Standard sound font, but Siesta features NES sampled drums, YM square bass, and a triangle/square double melody (a la Ironsword).
Goodnight is close to the Gimmick Standard font, only with the obvious exception of very wonderfully overt triangle wave harmony arpeggios. Also, just before the chorus, the bass briefly goes from NES samples to YM square before going back again right away. And then after the chorus, it goes back to square for a bit before returning once again to samples. Great interplay between the two bass modes. Also worth noting, at no point in this soundtrack is the triangle wave used as the primary driving force for bass, as it customarily is on standard 2A03-only stock NES/Famicom sound hardware. :-)
The very last track, the unused one, goes back to the standard Gimmick sound font. Nice way to round us out.
So, now that you've heard all about it, why not actually hear it? :-D
Gimmick, for the Famicom - who knew NES and Intellivision could sing such a lovely duet (though it's probably more accurate - albeit a whole lot less fun- to think of this as an NES/MSX duet instead).