Roland GO:MIXER PRO-X Owner's Manual page 18

(*6) If you want to record video using your camera app
or 4XCAMERA while playing back music from your
smartphone, turn the [LOOP BACK] switch "ON. "
(*7) Audio is output to the smartphone as two channels
(stereo).
(*8) If you are using the 4XCAMERA app, select some music
on your smartphone.
2
1
Tap and then
1
select the music on your
smartphone.
to
2
18