Which software bundle are you using to burn CDs?

Also, mp3s can become corrupted after being shared over and over again. The file my in fact be fine and intact, and you may be able to play it, but persnickity, bean-counting programs will call it invalid. I've had this happen to me before. All I had to do was use MusicMatch to convert it into a valid wave file, and I was able to burn it.

Hope this helps!