Try reinstalling the sound driver.
Start here:
http://support.dell.com/support/downloads/
If you can find the service tag, use that route. Otherwise click Select Model > Laptops > Vostro Laptop > Your model number > Confirm
Check that Windows Vista 32-bit is shown (unless, unusually, you've got the 64-bit version)
Click the arrow next to 'Audio'.
Click 'Download', then (if you're offered a choice) the lower 'Download' link.
Click 'Save File'.
Navigate to a suitable location for the file. (Your desktop is as good as anywhere).
Click 'Save'.
When the file has downloaded, double-click on it to run the driver installation program.
When that's been done, restart your laptop.
(You can delete the installation file then, if you want to).
Chris