Muddy Vocals, using AU-4fx with Sonar X1 Producer

Ok, I'm really liking this SonarX1 Producer, but for some reason I'm getting vocals that are really muddy?? loader or harder the vocal, the more mud I get, I've tried a Rode mic an M-Audio and others still getting lots mud, I'm using an older AU-4FX edirol interface for midi driver and capture, any ideas on how to fix these muddy vocals?, Cheers RJ