If I were to put an entire song in one sample, make a song with only that sample's note and then put it into a sega genesis game, would it work? (in a real genesis)
Short answer: Yes! You might need to plan it out heavily beforehand. And hassle with optimization.
Long answer: During the late 1980's, "Uncle Art" of European Amiga licenced game fame asked this question, and he had...
some results.
Basically he had a pre-recorded song and chopped it up. The song repeats parts, so Uncle Art had its repetition cut down so that instead of the sample itself repeating that part of the song, the Amiga
itself is repeating that part of the song in a sequence. This can turn what is actually a 20-second segment to a
4-minute song. The Amiga After Burner title theme was basically the grandfather of vaporwave's "chopped and screwed" technique.
Has it been done on the Genesis?
Yes. The tricky part of doing songs this way is figuring out how long your sample is and what sample rate (i.e. your "note") is going to be. You can have your song's "bars" be long before repeating, but that'll take up more space. You can have them be short, but that leaves little leeway for ways to repeat it. You can go for a higher sample rate, but it'll eat up more space. You can go for a lower sample rate, but that makes it more muffled and sounds nasty.
...of course, with the exception of sound driver hassles and sound driver limitations, there's nothing stopping you from doing this and
also having the YM3438's FM do some accompaniment. That's what
Combat Cars does, anyhow.