Unless the headphone jack is broken Asus chose to use a jack that doesn't turn off the speakers when a plug is inserted.
If the headphones aren't affected by the monitor volume setting then turn it all the way down.
I know that monitors with USB ports must be connected to a USB port on the PC first, a cable it provided for this. I wonder if its similar for an audio jack on a monitor?
Unless the headphone jack is broken Asus chose to use a jack that doesn't turn off the speakers when a plug is inserted.
If the headphones aren't affected by the monitor volume setting then turn it all the way down.
I just turned off the PC and it was fixed.
Now when I plug in earphones, the sound stops coming out of the internal speakers of the monitor.
I guess the monitor got confused when both the PC and the PS3 were connected!