I have the same thing here. When running windows, The sound only needs to be on 3 or so, but whit linux, I have to turn it up to 15 to be heard good.
I've runned redhat 8 for a while, and now I'm running slackware 9, but the both have the same problem.
And turning the mixer volumes higher doesn't help (well, yes the sound will get louder, but not as loud as whitin windows).
Dont know how to fix this, though.(Not that I really care; I just turn the volume louder when I run linux..