No sound to Dell 1720 speakers except thru headphone jack?
Onboard speakers are mute,only jack works?
Your speaker wires are probably broken. In their infinite stupidity Dell put the speakers in the lid and ran the tiny wires through the right-hand hinge where they're subject to flexing every time the lid is opened or closed. There is very little excess wire to deal with. Go to Dell's website and find the instructions for removing the frame around the screen and you may find enough wire to make a patch. But even your patch will be subject to wear every time the hinge moves.
Reboot the computer without the headphone jacks. Sometimes you can reset the power. Remove battery and unplug electric cord, Hold down the power button for 30 seconds. Reboot, if that doesn't work - Go to device manager and uninstall sound devices reboot.