Select an MP3 track with the same vocal track on the left and right audio channels.
Open the MP3 with the audio-editing software of your choice.
Click the track to highlight it.
Click the track name and select "Split Stereo Track." This will produce two separate tracks, one for the left-channel audio and one for the right.
Click the right-channel audio track to highlight it.
Click on the "Effects" menu. Select "Invert."
Click the track name for the left-channel track. Select "Mono." Repeat this step with the right-channel track.