the 32 bit Vista driver for the Dell Opriplex GX 520 & 620 is hard to find even when you know the name of the file you are looking for
There are lots of people looking for this driver without success
most don't say if they need the 32 bit or 64 bit version
some sites say the only option is to install a supported PCI sound card as Dell does not provide any Windows 7 drivers for this old model
The first useful link I found when Googling was this one below
it has a Link to a Dell ftp site
The Vista (32 bit x86) driver also works with Windows 7 (32 bit x86)
http://www.getpcmemory.com/drivers/download-dell-optiplex-gx620-desktop-pc-windows-xp-vista-drivers/
http://ftp.us.dell.com/audio/r145551.exe
(The Dell ftp link was tested today 28 March 2012)
this is what the driver readme says
Title: Audio: Analog Devices ADI 198x Integrated Audio Driver
Version : A25
OEM name : Analog Devices
OEM Version: 5.12.01.7010
Computers : Dimension: 2400, 3000, DE051; OptiPlex: 170L, 170L, GX270 / GX270N, GX280, GX520, GX620, SX270 / SX270N, SX280; Precision: 360 / 360N, 370, 450, 470, 650, 670
OSes : Windows Vista 32-bit Business,Windows Vista 32-bit Home Basic,Windows Vista 32-bit Home Premium,Windows Vista 32-bit Ultimate
Languages : Brazilian Portuguese, Chinese-S, Chinese-T, English, French, German, Japanese, Korean, Spanish
Created : Tuesday, November 06, 2007
you probably need to use 'Run as administrator' on r145551.exe
An alternative which I did not try is to go to Microsofts update site which will have 32bit and 64 bit versions
regards
Mike Barnes