Official comment
Serato DJ Lite or Pro? The DDJ-REV1 is a Serato DJ Lite controller -- if you want to use that unit with Serato DJ Pro, you would need to purchase an upgrade license to use Pro. For more information, see this article on the Serato website.
If you already have a Pro license or are having problems with Lite, check that your REV1 is not assigned as the default output audio device in the AUDIO MIDI SETUP app.