No sound on headphones


I have a Dell Latitude D620 with kernel 2.6.17-1.2145_FC5smp,and
surprisingly the sound is working (i.e. playing racks from a CD etc)!
But the sound is always played by the build-in speakers of the laptop
itself. When plugging in headphones, the sound still is played only
from the build-in speakers, but not from the headphones.
How can I switch that the sound is put out to the headphonse? I tried
around but didn't get it to work.

I appreciate any help in that.