Pioneer Dj DDJ-FLX4 Portable 2-Channel rekordbox DJ and Serato Controller (Graphite) - Manual (Page 118)

rekordbox for iOS/Android
118
En
6 Tap [Audio] > [PRE-CUEING] > [Split Output] to turn it on.
7 Press the headphones [CUE] button of channel 1 on the unit.
8 Tap [Audio] > [PRE-CUEING] to adjust the volume.
If stereo sound doesn't output correctly
If your Android device is connected with a USB cable, it may not
connect correctly and the stereo sound may not output normally. In this
case, set
[Android MONO/STEREO] to [MONO] on the rear panel.