I have a conexant card built into my laptop, and I must say it is the worst sound card I've ever dealt with, EVER. So regardless of whether or not it's the cause of the problem, I'd suggest getting an external card whenever you can afford it. If you take a look around on these forums you'll find tons of information on good cards.
Anyway, two things I would suggest checking. First off, is your main volume in cakewalk too far up? Try taking it down to about 30% and turning up your amplifier and see if that fixes it. I know you mentioned the V/U's aren't clipping, but sometimes I accidently set the master too high and completly forget about it.
The other thing is, is your output to your monitors coming from your headphone out? Headphone outs are powered outputs and many times preamplifiers don't like those at all and distort. If you don't have a line out on your computer, try going into your system properties for sound and taking down the volume as low as you can go while still getting sound. Then turn up your monitors again. If this fixes it, you can either leave it this way and lose a lot of sound range, or go buy an external card with a line out.
Hope this helps!