First issue, are you using the Dell supplied SigmaTel driver? If not, that could be the problem. I used to do a lot of support on the Dell forum and we found non-Dell sound drivers either do not work or do not work correctly.
Dell has a Vista driver and that should work OK on Windows 7 (Dell does not officially support Win 7 on this model). Drivers and Downloads | Dell [United States]
Download and install the Dell driver and see if that helps.