Why isn't the sound on some videos working?

This started a couple of days ago. I’ve noticed some videos don’t have sound on my computer, and some do.

This one does have sound (not interested in BigFoot people! Just clicking on random videos so you can see a pattern, if there is one!

This YouTube Video does not (but it has in the past.)

This video from UpWorthy, posted via fb, does not have sound either.

This one from ABC news,does.

I’m not adjusting anything anywhere inbetween finding these videos.