If you upgraded to Win 7 from XP, you need DELL Win 7 sound drivers. The XP are not compatible.
Dell has Vista/Win 7 drivers for the IDT Sound HERE
Also, keep in mind the correct and required install sequence for a Dell (1) Install Windows (2) Install Dell Laptop System Software (3) Install motherboard chipset drivers (4) Install device drivers including Video, Dell Sound Drivers
, Ethernet, etc. The "Dell Sound Drivers" are in bold letters as we have found, on the Dell forum, that non Dell drivers in most cases either do not work or do not work correctly.
If you still can't get it going, the best option is to post on the Dell users forum Laptop section so those familiar with the PC can help you get it going. Laptop - Dell Community