Check your manual FIRST.
Every keyboard I have had used the left out for mono...
This is all the Manual says:
Setting Up
NP-31/NP-11 Owner’s Manual
Make sure to properly connect all external devices you are using
BEFORE turning the power on.
A pair of stereo headphones with a 1/4" stereo phone
plug can be plugged in the PHONES/OUTPUT jack
for convenient monitoring. The speakers are automati-
cally shut off when a plug is inserted into this jack.
The PHONES/OUTPUT jack also functions as an
external output. You can connect the PHONES/OUT-
PUT jack to a keyboard amplifier, stereo system,
mixer, tape recorder, or other line-level audio device
to send the instrument’s output signal to that device.
So I guess that when I plug into headphone jack, it disconnects the power amp to the speakers and now I have Line Out. I've been looking for a NP11 schematic with no luck>