I have a method for making decent acapellas for mixes with nothing but a high quality mp3 or wave and Audacity.
1: Open track in Audacity. It MUST be stereo.
2: Split the stereo track into two mono tracks.
3: Set them both to mono and invert one of them.
4: Render it as a NEW Mp3 of the SAME quality/bitrate
1: Open the original Mp3 and the edited Mp3 together.
2: Hi-light the edited track, and go to noise removal. Sample it for noise removal.
3: Then Apply Noise removal on the main Mp3. You'll have to play with the settings here a lot.
If done properly, It will remove most of the sounds that aren't vocals, and leave you with a decently clean acapella.
I hope this helps c: