When we connect our laptop to external monitor, Linux OS in many cases cannot activate the external speaker in the monitor through HDMI. However, the following simple settings could make it work:

1. Connect your computer to external monitor through HDMI while you are using Linux

2. run the following command in Linux terminal to kill the running daemon:

pulseaudio -k

3. install "pavucontrol" by command:

sudo zypper in pavucontrol

4. configure your sound device with pavucontrol

Now, this should fix the issue of sound through HDMI under Linux.