On the C34F791 monitor, connected to a Surface Pro 3/4 (via mini DP) via DP on the monitor, or via HDMI on the monitor - audio is often "lost" even though enabled as the default audio device in Windows. It seems to happen only after the laptop is left sitting on for awhile.
A power cycle of the monitor is the only thing that brings the audio back.
Very annoying :-(
Still an issue - I have to power cycle it all the time. Tried with another Surface Pro 3 and 4, still happens. Tried a new miniDP to HDMI cable and miniDP to DP cable, still happens.