There is a nifty program which you can use to edit mp3 files. You can add effects, change formats, and cut/splice songs. It may not be exactly what you are looking for, but I've used it for that reason. It's called Wavepad. You can get it by going to
http://www.download.com/WavePad/3000-2170-10276213.html Its a small download, so you can get working soon.