I have an HDMI cable plugged in from my monitor to xbox 360, but every time I plug in the audio jack for headphones, the sound is still being produced through the monitor speakers.

How can I make my standard, non-gaming headphones work? I've tried plugging in the headphones into the back of the monitor but still, sound is being produced though the speakers.

What can I do or what adapter do I need for my xbox?

Most modern TVs have an independent setting for headphone volume, so you have to mute (or set volume to zero with the remote) the speakers, and enter the menu to change the volume of the headset. Could this be the case with your monitor?

Quote from Whiskey :

Most modern TVs have an independent setting for headphone volume, so you have to mute (or set volume to zero with the remote) the speakers, and enter the menu to change the volume of the headset. Could this be the case with your monitor?

I am using an actual monitor, not a tv screen. And I have no option to change the audio output on the monitor.