How can I cut the voice from a .mp3 song to get the background music?
There are plenty of utilities that you can use to remove the human voice from .mp3 songs. You can try Karaoke CD+G Creator, MP3 Karaoke, Karaoke 5 or Karaoke Sound Tools. Personally, I recommend you to use MP3 Karaoke because it is easy to use, free of charge and specially designed for .mp3 songs. After you download and install the software, open it and click the green + icon. Select your .mp3 song and press Shift + F3 key combination to begin the conversion process. After it is finished you can go to the output file by pressing Open Dir button.
It's handy to use an audio video cutter to cut the voice from your MP3 song or extract from video track. You can save the desired section as background music without quality loss in any format.