If you search the Dell site you will find the audio drivers. Unfortunately, the driver was released in 2005, long before Windows 7. This may mean the drivers are not compatible with Windows 7. Only one way to find out for sure... good luck!
I installed window 7 into my Dell dell optiplex gx620, but i cant find the audio driver
GX620 has the same audio (ADI 198x Integrated HD Audio) with OptiPlex 960, so go to the OptiPlex 960 drivers download website to get it (win7 32-bit/64-bit).
FYI Gents, For those of you looking for an audio driver, I downloaded and force installed this driver from Device Manager and and it worked On Win 7 64!. Windows Vista 64-bit released 8/23/2007 http://ftp.dell.com/audio/R145149.EXE