Since this has happened twice to a friend, I thought I’d document it for when it happens a third time.
A friend’s computer would play music, but not play the audio from a YouTube video.
When you left clicked the speaker icon, the volume was turned up and under the Control Panel the Audio devices were selected and working correctly.
I found the problem by right clicking the speaker icon and chose the “Open Volume Mixer.” It showed the graphic below. The sound was turned off for YouTube video and turned down for the System Sounds.
I’m not sure how this happened, but we’re ready when it happens again.