An Audacity user (Marco Diego) posted this technique on the Audacity Forum for using Audacity to attempt to isolate vocals; it will not work successfully in all cases.
It is sometimes possible to isolate vocals by using Audacity’s Noise Removal to capture the noise profile of a song that has had vocals removed, then run Noise Removal with that profile on the original mix before vocals were removed.
Make a copy of your original stereo track
Select the whole track (click in the space its Track Control Panel)
Copy the track with Edit > Copy
Create a new stereo track with Tracks > Add New > Stereo Track
Use Edit > Paste to make a copy of the original track in the new stereo track
Select the whole of the copied track (click in the space its Track Control Panel) 3. Remove the vocals in the copy by using Effect > Vocal Remover (for center-panned vocals)…
4. On a copy of the track remove the vocals as in Case 1 above using Audacity’s Vocal Remover
The copied track should remain selected, so use Effect > Noise Removal… and click on the Get Noise Profile button
Now select the original track (click in the space its Track Control Panel)
Use Effect > Noise Removal… and click on the OK button
Delete the copied track (click in the [X] at the top left of its Track Control Panel)
Behold. The absolute rock-bottom, least attractive part of producing a software program. Writing the manual. If you do this much more, we’re going to invite you to join the team. Be careful what you wish for.