If your keyboard has speakers you wish to disable while monitoring to the line outputs, then putting an unconnected jack plug in the headphone socket is a harmless means of muting them.
I have a Casio PX310 digital piano. When jacks are inserted into the (quarter-inch) line out sockets, the onboard speakers are not muted. They are muted when headphones (or a jack) are inserted into either of two 1/8-inch headphone sockets. The line out volume is also controlled by the onboard speaker/headphone volume control, so it’s not an option to just turn it down. So, to mute the onboard speakers, is it safe to just insert an open-circuit stereo jack into the headphone socket, or would it be better (for the onboard amps) to present it with a dummy load? Elsewhere I read that a 100Ω resistor from tip to sleeve and ring to sleeve would do it, but I don’t believe anything unless I hear it from the SOS experts!
SOS Forum post
SOS Technical Editor Hugh Robjohns replies: Yes, a dummy jack plug or adapter will do just fine — and it won’t cause any problems!