On Linux systems using NoMachine v8.16.1, the client is unable to mute HDMI-connected speakers through the NoMachine user interface.

Audio continues to play on the server side even when muted from the client.

As a workaround, check for the status of the IEC958 channel by running:
amixer get IEC958

If the command returns any information, it indicates that the digital audio output (commonly used for HDMI or S/PDIF) is available.

Mute with command:
amixer set IEC958 mute

Unmute with command:
amixer set IEC958 unmute

Use this alternative method if the above fails: disable and enable the output with the following commands:

amixer set IEC958 off
amixer set IEC958 on

Trouble report solved in version 9.1.24.