Yeah, I noticed Mansell's absence too, but then looking at the soundtrack he's only credited for two tracks; 'Leaving Earth' & 'An End, Once and For All'. Makes sense though as those two basically bookend the game, but the latter has a co-credit with Hulick, so...yeah. He's more of a guest contributor really.
I noted in part two one of them mentions how the music wasn't always used as intended in ME1. I'm assuming the main culprit would be the Benezia death scene. To this day I'm not sure if that was ineptitude or just some glitch they never got around to fixing. If it's the latter I'd really like to see that scene with the intended track.