Are you using the Dell supplied sound driver
or a driver from somewhere else? I used to do a lot of support on the Dell forum and we found in most cases the Dell sound driver was required. Non-Dell drivers
either did not work or did not work correctly.
You can download the Dell sound driver from www.support.dell.com
use your service tag number to access the downloads. If you would have provided the Vostro model number, I could have linked you directly to the correct download.