Sound drivers for dell optiplex 745

No Windows 7 Driver for your Dell, but the Vista Driver should work. Download and SAVE the correct version (64/32bit) from here
http://www.dell.com/support/drivers/uk/en/ukdhs1/Product/optiplex-745
Once downloaded Right click on the Driver Setup File in your Downloads Folder and choose 'Troubleshoot Compatibility' Windows will then get you to try installing with XP settings. Choose 'Test Program' then 'Cancel' as you want Windows to try the next settings which will be Vista. Having clicked 'Next' and 'Program worked in a previous version of WIndows' choose Vista and it should work when you click 'Test Program'