If I understand Letha correctly she's doing this:

AKG mic -> Behringer Mixer w/USB out -> Audacity

I think the mixer is the culprit. I would agree with the person who suggested that you might be setting the gain too high on the mic input on the mixer. You might also have too much treble and midrange on the EQ section. The most likely reason that you would have too much treble and mids is that you've got the bass turned up and need to compensate. Think twice about turning those EQ knobs past 12 oclock, your mixer will start to ADD sound to your recording. When it comes to EQ, less is more. Many of the gurus on mixers only use the eq to SUBTRACT, not ADD, believe it or not. Hope that helps...