How do I reset the audio system when audio can't be heard anymore Playing music on YouTube in Chrome did not output any sound. Playing video files with Quicktime and VLC didn't work. I noticed this problem before and it fixed itself after a restart. But I wondered if I could reset a process to get audio back.
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.