You can check the Software -> Editing secion of this site; there should be at least one piece of software there that can add voice or sounds to a WAV file.
It's best to work with WAV files when you're doing heavy editing, so I recommend you use RazorLame to decode the MP3 to a WAV which you can do the editing with, and then re-encode to MP3 (re-encoding will cause some quality loss, but if you're using a good source file and proper settings, you can minimize this.)
Removing vocals is more challenging; removing anything from a CD track, WAV, or MP3 is like removing the eggs from a cake after it's been baked. Well, actually, it can be done, but it makes the rest of the song badly distorted, and chances are the vocals won't be compltely removed anyway. Anyway, you can try a vocal removing plug-in for the player WinAMP. But I think you'll agree with me that the sound quality of the song suffers.