How do I reset the audio system when audio can't be heard anymore
Killing the coreaudiod process might work (it did for me). One way to restart it is to launch Activity Monitor, then View->All Processes, scrolling down to coreaudiod then clicking the big X button to force quit. It gets relaunched automatically.

Another option is at the command line:

sudo killall coreaudiod


After doing that and refreshing the Chrome tab - audio output works again on my macOS High Sierra 10.13.6. Source of idea.