Using a test tone and oscilloscope would of course be nice, but since you probably don't have an o-scope you can select AC on your DVM and place the ground lead on the ground pin " of the 6.35 headphone jack. Play a test tone and then place the red lead on left and then the right channel pins. You will read an AC voltage when playing the tone, both channels should be almost equal. If you have voltage on the input side of the socket when the headphones are plugged in and nothing on channel of output side of the 6.35mm socket your problem is with the socket. If you don't have an AC voltage on either the input or output side of socket on one channel you issue is before the output socket. I don't have one of these jacks but if memory serves me correctly I believe the ground is the pin farthest from the front panel. "verify" These jack's are switched, so when nothing is plugged in you will not read continuity across the pins when you plug in a jack you will read close to zero ohms across the jack. "no power applied of course".