Hi
the problem is that if you go to Dell there are no Win 7 sound drivers for Optiplex GX 520
however the Sound Drivers for Vista (+ XP) also work with Win 7
Sound / Audio Drivers for GX 520 & GX620 are the same.
You have to specify 32bit or 64bit version
The links to drivers at Dell can be bound at toms hardware
regards
Mike Barnes