Good point, it may well be cured by reducing the burning speed down, especially if your PC is a bit on the slow side. Most burners burn the CD at the slowest speed necessary. The black CD is written from the inside out (oposite to a record!) and the inner part of the CD is revolving at a slower speed than the outside edge, therefore when you are writting at say x52 (the physical maximum speed) the first few tracks are written at about x 16, the middle part at x16 - x32 and only the very last couple of tracks actually reach x52 speed; that's probably why the tunes get worse from the middle to the end.
Good idea to invest in some decent quality blank CD too