"Let All Mortal Flesh Keep Silence," like "O Come O Come Emmanuel," is not a Christmas song but rather an ancient hymn. And like the other, it is just best sung in church. This is the only halfway decent video I could find of it. I find the scriptures on the screen annoying. Fitting they may be, and certainly worth reading and pondering, but I find it a distraction when I want to focus on the song. The lyrics can be found here.