Hello everybody, if this is still a problem...
I have had this problem since I have bought my computer (Desktop, Asus P5GC-MX motherboard)
And I've had the computer for over a year it's just tonight it started angrying me
So I've tried every single solution in this thread, but somebody mentioned towards the start of the thread about the BIOS. I don't know if I have the same problem as all of you lot, you all seem to have Acers. I tried every patch and driver update available (I'm using Vista Ultimate) to no avail.
So I rebooted into my BIOS.
Under advanced there was an output devices selection, and then audio. It said that I had HD audio selected for front and rear. I changed the front output to the only other available option (Which happened to be "AC97") Saved and booted. Presto, it works. Then some small tinkering in the Realtek Audio Manager (So that the headphones and speakers play the same stream at the same time) And when I want to listen to my headphones I just turn the volume down on my speakers (physically) and plug the headphones in.
I hope this post may help other people
(I registered just to say this)