[1764 is a dell inspiron model number
When you plug in your headphones, sound from the speakers should cease, which is normal. Does this still occur?
I would take a look to see if you have the latest audio drivers
. Head to Dell's website and download the latest Realtek driver
. Drivers and Downloads
What's listed there is from April 22nd of this year so I would definitely take a look.
Let me know how that turns out. if it doesn't work we can go from there!