Why is mute not working correctly on my monitor?

A monitor can support DDC volume adjustments while ignoring or mishandling its separate mute command. Working volume therefore does not guarantee working mute or unmute.

A DDC volume command sets the volume level. A mute command turns mute on or off. Setting volume to its minimum is a different operation and does not guarantee silence on every monitor. Likewise, raising the volume may not release a separate mute state. Some monitors need different values for turning mute on and off. Others need the volume and mute commands sent in a particular order.

Follow Fix mute and unmute on an external display to choose compatible mute/unmute methods, check minimum-volume behavior and adjust DDC mute values if needed. If only the native keyboard mute key is affected, the tutorial also explains how to leave that key to macOS.