[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!