Audio driver

Dell Drivers & Downloads  Enter "audio driver" in keyword box.

My previous 755 used SoundMAX.  I didn't immediately see a name for the audio driver for your 780 on Dell's site.  Of course, make sure there's no other audio drivers on your PC first.  There won't be Win10 drivers for your 780.  You may need to select "other" depending on your OS.  I don't know that your 780 takes a Realtek audio driver, but I do know the Optiplex generation newer than the 780 does.

A SoundMAX driver did come up for it.  Read directions.  Softpdedia SoundMAX  Dell site is probably safer bet.

If by chance the driver from the Dell site doesn't work, you may need to reload OS to clear out any 'residue' from other drivers.

Also:  https://www.dell.com/community/Desktops-General-Read-Only/Drivers-for-Optiplex-780/td-p/3580207 

If by chance you have no luck, please let us know.  It helps to know what OS you have and what the situation might be that there's no audio driver, like a recent OS reload.

following the link already noted in first reply, the audio driver for 780 is

Analog Devices ADI 198x Integrated HD Audio

same driver for both Windows 7 and vista 64 bit, which are most likely compatible with Windows 10 64 bit.

https://www.dell.com/support/home/en-us/drivers/driversdetails?driverid=r297758&oscode=w764&productcode=optiplex-780

In case I caused any confusion, I guess "Analog Devices ADI 198x Integrated HD Audio" is the name for it.  Was thinking I'd see something different.