so,I have an xbox 360 plugged into my monitor with HDMI cable. My monitor audio is plugged into my PC with 3.5 mm jack.
My headset is also plugged to my PC with 3.5 mm.
Is there a way to use my headset for the xbox the way it's plugged in?
Without plugging the headset into the XBox, it won't work. You can purchase "Y" cables that will merge the sources, however, the signal could damage on or both of the devices.
Best bet - get a switch box like this: http://electronicsusa.com/mk1and2.html