For those who are trying to find bgm files for recording their own voices, Audacity actually allows removing of vocals from songs in mp3 format.
1) Open the mp3 in Audacity
2) Look for the black arrow to the left of the blue color 'soundwave' (above where it says 44100 Hz)
3) Select 'Split Stereo Track'. You'll see that the blue soundwave sortof 'splits' into two.
4) At the bottom soundwave, click on 'Solo'. The top soundwave should be faded out.
5) Highlight the entire length of the bottom soundwave (the unfaded one)
6) Go to the menu at the top, and select 'Effect' -> 'Invert'
7) Click on the black arrow for the bottom soundwave again and select 'Mono'
8) Click the 'Solo' button for the bottom soundwave, the top soundwave is now unfaded.
9) Highlight the entire length of the top, and click the black arrow. Select 'Mono'.
10) Save file.