No sound via USB-C to HDMI adapter connected monitor speaker

I have a MacBook (Retina, 12-inch, Early 2016) running macOS Sierra (Version 10.12.2), which is connected to an external monitor via Apple USB-C to HDMI adapter.

For many months, it worked well for both picture and sound, but it all the sudden stop outputting sound to external monitor. Picture is continued to be displayed OK.

I had an exact same issue about month ago, and in that time, I was able to get back sound when I opened "Audio MIDI Setup" application, and when I was changing setting of build-in microphone format frequency from default to some other frequency, and then bring it back to original setting.

This time around, this trick did nothing to sound. In "Audito MIDI Setup" application, I have "DisplayPort" panel, which corresponds to my monitor with speaker. In this setting, I can see that "Master Stream", "1", or "2" are all set to zero (i.e. round mark is all the way to left.), and I have no ability to change this setting. Unfortunately, I don't really know how this panel looked like when sound was coming out of monitor speakers just fine via HDMI adapter connected monitor, but my guess is that this "master stream" should not be set to zero?

I would appreciate if someone can help resetting this... I have also already tried restting PRAM, restarting with "Safe Mode", etc., but nothing helped so far...