I have AOC u2868pqu and no sound coming out of the built-in speakers (audio-in cable in Audio-in port). I found out the speakers only start working when PIP/PBP is turned on (even with nothing as secondary source) and Audio is set to ON in PIP settings. speakers become mute again as soon as PIP/PBP is turned OFF.
I contacted Tech Support from AOC.
In the meantime I figured out that when HDMI/DP is used Audio-In is disabled. It can only be used with VGA/DVI.
With HDMI/DP user is forced to rely on HDMI/DP audio since there is no option to force the use of Audio-In as the audio source like on most monitors from other manufacturers.
I asked Tech Support for a firmware update to change this. They replied something equivalent to "Yeah right, keep on dreaming kiddo" ... Well ... It was worth trying, I guess...
So DP audio is my only choice apparently... Since I'm on macOS, it sucks cause I cannot change the volume from within the OS (macOS thinks I'm using a TV and thus I have a remote to adjust the volume)... This has been a common issue for years. Apparently there is no reliable third party app to fix that. All options are either buggy or suffer from delays/parasites or both.