In zsh:
autoload zmv
zmv '(*).MP3' '$1.mp3' # rename files in the current directory only
zmv '(**/)(*).MP3' '$1$2.mp3' # rename files in subdirectories as well
In zsh:
autoload zmv
zmv '(*).MP3' '$1.mp3' # rename files in the current directory only
zmv '(**/)(*).MP3' '$1$2.mp3' # rename files in subdirectories as well