My DJM 400 one of the outputs is louder and has distorted sound what do I do?

First verify that one channel is bad by swapping the output leads and seeing the bad audio comes from the other speaker. Assuming it does, you probably have a bad output driver on one side.
This can happen if one powers part of the system from different receptacles in a venue and electrical disturbances destroy one or more audio interconnects. ALWAYS power ALL interconnected equipment from the same source, even if you have to run an extension cord between power amps and the mixer.
You will probably need to take the unit for repair. The parts cost should be nominal, however labor could cost a bit.
Find out is SAME distortion is present in the headphone output. The circuitry shows a bunch of muting transistors and one of them could be bad... but we can do some troubleshooting externally with testing at headphone jack, etc.