Altering an mp3 track
If you simply want to overlap the voice, you can do that by recording using any free softwares available on internet (just google free recording softwares)
you can try, cooledit as well, one track will contain your file with play mode, one track with record mode (for your voice)
then start recording, it will play the song simultaneously with recording your voice.
After then, you have to save the mixdown.
other long way is, you have to get KARAOKE for that song (karaoke means, no vocals only music track) then you'll get the recorded song in good quality.
Hope this helps.