If it looks like this....
https://www.amazon.com/ASA-JDVD1500-...27922336&psc=1
....then these are the only instructions I found in the manual...
"Audio Menu
Press the
volume control knob 3 ) to access the audio menu. You can navigate through the
audio menu items by pressing the volume control button repeatedly. Once the desired menu
item appears on the display, adjust that option by turning the volume control knob within 5
seconds. The unit will automatically exit the audio menu after 5 seconds of inactivity. The
following menu items can be adjusted.
Bass Level
Rotate
the volume control knob 3 ) to adju st the Bass level range from “ 7 ” to 7
Treble Level
Rotate
the volume control knob 3 ) to adjust the Treble level range from “ 7 ” to 7
Balan
ce
Adjusting Balance controls the relative level between the left and right speakers in each pair.
Rotate
the volume control knob 3 ) to adjust the Balance between the left and right speakers
from “R 7” (full right) to “L 7” (full left). “0” represents an equal balance between the right and left
speakers.
Fader
Adjusting Fade controls the relative level between the front and rear speaker pairs.
Rotate the
volume control knob 3 ) to adjust the Fader between the rear and front speakers “R 7” (full rear)
to “F 7” (full front). “0” represents an equal balance between the front and rear speakers."
https://www.asaelectronics.com/downl...ers_manual.pdf
It looks like you might have to set the outside speakers at 0 to turn them off with the fader or balance.