Thanks for both of the replies, but neither helped. This problem, as I mentioned, if video only, audio plays normally, so it's not a windows setting. And in MC21, that checkbox does not appear there.
Also, I feel like I've tried every setting there is, but for whatever reason, audio (music and spoken word) plays fine, but video files play at about 1/10th the volume.
Since no one else is seeing this behavior, it's not a flaw in MC21, so it has to be a setting I have set. Does anyone know enough about all of the audio settings to guess which one would be making video so low, but regular audio regular?