Dedicated PC gamers have often trumpeted the superiority of their chosen platform in part because of the ability to expand a PC's capabilities with add-on upgrades. Their claim was that home consoles had a static hardware configuration and thus could never be upgraded without buying a completely new system. And they would be WRONG.
Cartridge-based systems like NES and SNES could indeed be enhanced via expansion chips installed directly on the cartridges themselves. The NES in particular had Memory Management Controllers (MMC), in both first- and third-party varieties. Among the improvements provided were additional sound channels to the measly five in the base hardware. I'm sure you are all aware by now of Konami's VRC6 chip and how it made the Japanese version of Castlevania III sound far more spectacular than the American version.
Chiptune artist Xenon Odyssey recently released his debut album, Fountainhead, a pay-what-you-want tribute to the entire scope of NES sound. Composed in FamiTracker, Xenon used just about every audio expansion from the hardware's life with the sole exception of Sunsoft 5B -- which was only used in a single game, Mr. Gimmick. A wonderful sampler platter of the NES' audio range, Fountainhead is the shizz and you should give Xenon a high five for his solid work.
Fountainhead [9-bit Records]