No usb sound output for CH1 and 2

I have a new MC7000 which was working great with Serato DJ for about 10 days, then suddenly the USB sound output on channel 1 and 2 stopped working. The channels work fine on line, and channels 3 and 4 still work fine with USB and line. The midi controls for CH1 and 2 are still responsive; tracks load and trigger but there is just no sound and no signal indicated on the CH1 and 2 LED level displays. All filters and software FX are off, everything has been rebooted repeatedly. Airport and bluetooth are off, and every other connected device removed from the computer, tried a different USB cable and tried the other USB slot on the Denon. (I’m on Mac OSX 10.11.16)

Try testing with VDJ, if that shows the same behaviour then its a hardware issue

Do you mean the Serato Video plugin? When I load a video file with audio, decks 1 and 2 control the video but produce no sound output, decks 3 and 4 play only the audio track from the video files (and do output that sound to mixer). I’ve never used Serato Video before so it’s not immediately clear to me if that is normal. I would think decks 1 and 2 ought to output the video’s sound track if it is present?