There are two sound chips in the Megadrive, the non-FM chip is the same as the audio chip in the Master System (and BBC!), and has three square wave channels and a white noise channel. Playing back samples is achieved using the volume register on this chip, in the same way that you would get the C64 to play samples. It produced 4-bit samples.
As to the SNES chip: Nightmare. I hated it. It's the precursor to the Playstation and PS2 audio chips, which are also both dreadful. It only has 64k for both player code, music data, and samples. The samples are compressed into 4-bit, which effectively tripled the RAM, but that killed any chance of doing real-time synthesizing. If you wanted the reverb in there (actually a very simple delay unit), that took up more RAM. That means that the samples really had to be miniscule. Not only that, but because of the compression, you could only set loop-points on 16-sample boundaries. It was an arse. More RAM could have easily solved it. The Playstation and PS2 both suffer the same RAM problem, but have CD drives.