Drop .ogg files into 'Regular Battle' and 'Heavy Battle'.
The song will be picked at random if there's multiple files.

IMPORTANT: files must be Ogg VORBIS. Many .ogg files from YouTube
rippers / online converters are actually Ogg OPUS and will not play
(the game log will tell you when that happens). Convert with:
  ffmpeg -i "input.ogg" -c:a libvorbis "output.ogg"