For awhile now, we've been getting complaints about YouTube videos not working on some of our computers. The problem has been easy to reproduce, because, sure enough, the video will play for a second and then the viewer window goes black/staticy and displays a "video cannot play" error.
Usually the culprit is an out-of-date browser or plugin, but even with the latest versions the problem persisted. Searching online didn't really turn up anything useful. I got so frustrated that I had to go to my second-to-last-resort, YouTube's help suggestions - still nothing.
Finally, I went to my last resort, asking our IT person to take over the problem because I wasn't making any progress. It baffled her too for a little while, but after some more online searching, she came up with a fix: plug in the headphones.
I believe our computers automatically mute their speakers when headphones are unplugged (so as not to play sound otherwise, which may bother the patrons around them). YouTube must be looking at some sound setting in the computer and doesn't like not having available speakers, and so just doesn't play the video at all. But if you have headphones plugged in before the video starts, it works just like it should. I have no idea why YouTube videos would not play without this, but there you go.
I don't understand it, but it works, so maybe I don't need to. Just, thank goodness for smart coworkers.