How do I control volume on Displayport monitor with the keyboard master volume?

Apple do not make it easy to control the volume on Displayport, USB-C or HDMI connected monitors. All of these run the audio in digital form over the same cable as the video. For this type of connection with digital audio Apple take the view that the playback device i.e. the TV or monitor should be responsible for adjusting the volume.

This is because if you adjust it on the Mac the following has to take place.

Digital source -> Convert to analogue -> adjust analogue waveform to change volume -> Convert back to digital -> send to display -> display converts to analogue -> sound played via speakers

Whereas if the display is responsible for adjusting the volume the process is as follows.

Digital source -> send to display -> display converts to analogue -> adjust analogue waveform to change volume -> sound played via speakers

As you can see the later Apple enforced approach removes the need for an additional set of analogue/digital conversions. Besides preserving more of the original data - all conversions like this degrade the signal, it also prevents adding additional latency which at worst can result in the audio becoming noticeably out of sync with the video.

For HDMI displays there is a better approach which sadly Apple so far do not support which is for the Mac to send CEC commands down the HDMI cable to tell the display to adjust the volume. CEC requires hardware support on the HDMI interface. You can however add this ability by buying one of these https://www.pulse-eight.com/p/104/usb-hdmi-cec-adapter you also need some software, see http://couch-slouch.com/compatibility/

For Displayport you might be able to use a different approach, this involves redirecting the sound through another program before it leaves the Mac. See - https://lifehacker.com/how-to-enable-mac-volume-control-for-hdmi-or-displaypor-1 793607216

Hi,

Connect Mac Pro Line out port to monitor audio input port with mini stereo plug should works.

You need to set System Preferences > Sound, select Line out as sound output.