The first one was one I resampled with VLC. I don't remember what codecs I chose -- it appears on Firefox as a sound-only file. The second one was a .mp4 file I saved from somewhere a while back -- a kugelblitz event. It shows up unchanged from the file I saved and uploaded.
It is easy to make a mess of a video with VLC, merely by choosing odd combinations of codecs and file extensions. VLC will play anything, but most combinations do not show up in a browser.