If you want to test whether the speakers are going bad, remove your Realtek audio driver.
If you have Windows 7 go to Start>Control Panel>View by Category>Uninstall a Program. Go down the list to find the driver and uninstall it. Restart the laptop and Windows will install its own native audio driver. If you still hear the crackling with the native driver -- only through the speakers and not the headphones -- then the speakers are failing.
To get rid of the native driver just download a fresh copy of the Realtek driver from Dell and install it.
If you determine that the speakers are going bad and if you are still under warranty, contact tech support to schedule a repair job.
Jim Coates -- 15 years on the Dell Laptop Audio boards -- since 2/6/04