
I don't understand why you seem to be implying that the DJM-900 Nexus would have any effect on whether or not your computer can properly communicate with the CDJs for HID/MIDI controller mode. If you're trying to use the USB sound card capability on the mixer, it's totally independent of the CDJs, as is the CDJs' USB sound card capability vs the mixer. It's possible that no USB sound cards were active in Serato (the mixer or the CDJs), which might explain why nothing would play but you sorta kinda had HID/MIDI communication happening.