There is a small contact in the headphone jack that is supposed to mute the PC speakers when the headphone plug is inserted. Have you done anything to the jack?
I have a Dell XPS_XPS 625 running windows vista premium 64 bit (SP 1). AMD Phenom(tm)9850 Quad-Core Processor 2.46 GHz. 4.0 GB RAM. When I play or listen to music, a game that I am playing or any other audio my speakers still play although my headphones are plugged in. Help. This is very serious. My wife is very upset with the sounds of gunshots (CoD World at War) after 10:00 p.m.