The Megadrive sound was quite odd...to me there were two types of sounds in the Megadrive. The music, which sounded a lot more synthetic than on the SNES, but fine and quite nice in some games, and the digitalised voices (which were a thing) and sounds, which to me sounded very mediocre. It is as if the Megadrive had 2 different sound chips.
That's exactly how the mega drive was. It had 8 FM synthesized voices, and one voice for sampling digital sounds, which was very limited. The SNES in comparison simply had 8 voices for digital samples, so it was more versatile in the timbres you could get out of each voice, but they were constrained by the compression. Ironically, a lot of games were using very synthy samples on the snes, partly because they were popular back then, and partly because they were cheap to produce.