Dell does not support Vista/Win7/Win8 on this model. XP is the latest OS. The sound in the GX620, ADI, is obsolete and apparently not supported by Win 7 and the generic basic function HD Audio Codec. Windows XP drivers are not compatible as Vista/Win7/Win8 handle some sound functions differently. You will probably have to do what others with Dell's that are not supported have done, install a PCI sound card that is compatible.
I used to do a lot of support on the Dell forums and that's what users were doing with incompatible systems.