What you should do is use a multitrack recorder like Vegas, Acid, or even Cubase VST.

This way you will select one track, record whatever you want, and then select another track, and record another source ont his track, etc... and you will finally have several tracks playing together. You can even separate them later on, or play with the volumes on the software mixer.

This way you don't damage the other recordings and always have them handy for tweaking

Most of these programs let you import MP3s or WAVs directly int he program too.

Good luck