Support for the DDC mute command heavily varies from display to display. BetterDisplay offers various options to maximize compatibility.

Please note that if DDC is generally not working or Volume control is not working with the display (some displays generally lack support), then DDC mute will not work (FIX 1 should still work with that for mute).

The following methods can be used to fix mute:

UPDATE: See the comment below for updated screenshots for the v2.x and v3.x versions!

FIX 1 - Simply leave the mute key to the OS

If you mainly want to use the native mute key on your Apple Keyboard, you can let the system handle mute as usual (cut off the digital audio stream to the display). You can set this up by enablig the following setting under Settings/Keyboard:

FIX 2 - Change mute, unmute methods

BetterDisplay supports various mute and unmute methods. Some displays don't like the DDC mute command but will respond to setting the volume to zero. Other displays do not like the latter, but only respond to the mute command. Other displays need both in specific orders. Extra problematic displays might need special treatment for unmuting as well.

You can change the mute/unmute methods under Settings/Displays/Show DDC audio settings (you can set these on a per display basis):

FIX 3 - Change mute values and mute command code

There are some displays that require non-standard DDC values for mute ON and OFF, or need a custom control code to access the mute functionality. You can experiment by changing these as well under Settings/Displays/Show DDC control settings (you can set these on a per display basis):