When I connect to DDJ-FLX4 with a USB cable, the sounds from the headphones and the master are combined in the master output of DDJ-FLX4. There is no sound from the headphones.

We have confirmed that 2-stereo outputs cannot be done on some Android devices.

By setting Android MONO/STEREO switch on the rear panel of DDJ-FLX4 to MONO, the master and headphones will be output in mono.