Thanks for posting.
One of the Rockstars answered this a while back. Here is what they said:
Check the Dell downloads at www.support.dell.com for any updated sound driver. Also, make sure you are using the Dell Realtek ALC269Q-VB3 sound driver and not a generic Windows installed driver, or a non-Dell Realtek driver which we find does not work properly. Check the Device Manager and if it shows "HD Audio Codec" that means the generic Windows installed sound driver is being used.
Only DISPLAY PORT ++ has sound when using DP to HDMI.
ALL HDMI has audio but only Display port ++ has audio.
The audio on Display port is optional.
The rear on my 580 does not show display port ++
only regular Display port WITHOUT audio.
Also the Optiplex 760 and 780 do not have display port ++ audio.
This is incorrect.
"Display Port cannot transmit audio, only HDMI and DVI can do that."
DVI cannot transmit audio. Both HDMI and DP can transmit both video and audio signals.