Happy to announce that one of the game's most requested features has now been added: the ability to play your own music in-game along with the disco light visualizers.
Instead of manually-inputted beat mapping, the game runs a real-time audio analysis on the custom music and uses that to drive the lights. It's not quite as note-perfect as the manually-inputted data but still feels pretty responsive.
It should be easy to set up - go to Options / Audio and then paste in the path to a folder containing music files. Example:
C:\Users\Erik\Music\Dodge
Make sure you hit Enter after typing or pasting this in! Then select the 'folder' option button instead of 'soundtrack'. You can swap between custom folder soundtrack and the built-in game soundtrack at any time.
The one restriction - and it is a bummer - is that playback only supports .wav and .ogg files. This is based on a Unity restriction that I can't work around. .wav files are uncompressed and very large. .ogg are compressed but not super common, however I was able to use a tool like this one to convert and mp3 to an ogg: http://audio.online-convert.com/convert-to-ogg (be wary, that site has a few fake 'download' buttons that you should ignore).
This thread has some more discussion on the best ways to convert your files: http://steamcommunity.com/app/270450/discussions/0/617329505857631477/?tscn=1425419665
Enjoy!