RE: Dell OptiPlex 755 No Audio Output device and no Sound/Audio in Device Manager
There could be multiple hidden audio devices.
* Press windows key + R
* Enter cmd, click OK
* Type cd\ [press Enter]
* Type set devmgr_show_nonpresent_devices=1 [press Enter]
* Type devmgmt.msc [press Enter]
* Once the Device Manager opens, open Sound and see if you have multiple grey out items. If yes, delete them, then restart the computer and retest