oh boy!
ok so Mazedude has a good list to start with. here's a few more, while also promoting my own youtube channel a few times where i chronicle these things.
Jesper Kyd (Soundwave): demo-ways, probably best known for Hardwired, 2nd place in the amiga category at The Party 1991. nowadays, best known for Assassin's Creed, but he has some VERY demoish early work you should dig through!
Martin Iveson (Nuke / Spaceman): I don't think he scored anything on the scale of Hardwired, but he was fairly active in Anarchy (who organized The Party for a while) and Lemon (iconic group). He was part of Core Design, a dev group founded by demosceners. He left a huge impact on the Amiga (seriously check out Wonderdog and tell me you don't recognize SOMETHING). He's know today as Atjazz, writing some banger chill-house type tunes.
Matt Simmonds (4mat): I pretty much associate this guy with the shortlooped waveform Amiga MOD chiptunes, but he's been scoring games for just as long.
Jonne Valtonen (Purple Motion): One of the musicians for Second Reality, perhaps the most iconic DOS-era PC demo. He moved into scoring games a few years later. He was even credited as Purple Motion in Rampage Puzzle Attack for the GBA.
others?
I can't remember off the top of my head, but I think several demoscene musicians got into writing GBA tunes. It turns out that hardware was a perfect match for their skillset.
there's also a few other tenuous connections. For example, Barry Leitch and Jason Page made a few C64 demos as teens, but those demos didn't really spread much further than local computer shows.
!!! That was Neil Biggin!
I love that dude because he was living right next to Warp Records and it SHOWS in his style. I'm surprised he didn't end up at Warp.