First Solution (Many kb saved , lower quality music)
Import the music into audacity , set hz to 96000 > Export as something ...
Import the exported and again .. set the hz to 96000..> Export
Import to world editor .. use as sound -> set sound to pitch 0.20
Test the sound and change the pitch until the sound you liked..
Second Solution ( Lower kb saved , no change of quality)
Look to the end and the start of the music , if there is a fade in place that is empty or very little sound ... delete it , if it doesn't have any .. use first solution ..
HINT : I deleted a little bit from a music and it became 6161kb to 3XXXkb