While this problem is not common it happens when there is a
mechanical fault with the outlet where you plug in your
external speakers. Within the outlet is a small switch which
when a plug is inserted, turns off (opens) the connection to
the internal speakers. When you remove the plug from the
jack (outlet,) this switch is supposed to close to permit
the internal speakers to work. If the switch gets hung up
or breaks no signal can go to the internal speakers. If the
unit is under warranty, it is time to return to Dell. If
not under warranty, any decent repair shop should be able to
replace the outlet/switch combination but it probably will
require dismantling of the laptop and soldering on the
motherboard. It will also probably be fairly expensive.
Your choice, to either have repaired or to leave as is and
use external speakers. Small, non-powered ones can be had
for little money. Battery/shore powered ones are slightly
more.