Sounds like the software that you're using isn't mixing very well with your
system. No one here could really tell you exactly what the problem is
without troubleshooting a good amount. If you are getting the same results
using different brand new blank CD's, and since it seems to get progressively
worse, I would have to go with a crappy burn program or misconfiguration.

The first thing that I would try is to download musicmatch or some program
that will do this for you. I actually had a similar problem and no matter which
software i used, it was like the problem changed slightly but was still there.

I imagine the MP3's sound find before burning? do you have another brand of
blank cds that you can try? The bottom line is you should try other software
first IMO.