More Video Tutorials Here
To save as mp3 you need to have lame mp3 encoder installed
This works with stereo tracks that have the vocals centered, when inverting a track, the waveforms cancel but it may grab more than vocals in some cases. Below are more alternatives.
Save to one track at the end. Reduce the audio volume with amplify to make the softest parts disappear then increase it again. This will remove the vocals completely so quit whining about the "reduction".
Enable VST in Audacity for More Plugins
Audacity Nyquist Plugins
Goldwave has High/lowpass filters for vocal reduction