Title says most, but let me elaborate. If you can't be bothered to read skip the next 2 paragraphs.
Recently, I tried to put headphones to listen music and talk with other people without disturbing anyone. With headphones I didn't notice at first, but then someone pointed out that sound was still coming out of the speakers. This annoyed me but I had to deal with it.
Yesterday, I tried to connect something to my PC, but about 99% of the sound was still coming out of the speakers. Turning up the volume to the maximum made a very small amount of sound to come out by the speakers connected on my PC (not the build-in ones) and a lot still come out by the build-in ones.
To sum it up : When I connect headphones next to nothing comes out in it but it still comes out of the PC's speakers.
Windows 7 64 bits (laptop).
ASUS AMD Radeon 6570M/5700 series
Intel Core i7 CPU, 1.73 GHz (8 CPUs)
About 13 months old.