Here are the results now that I have installed my full AV stack. They are somewhat disappointing.
mp4 - plays in firefox
mkv - offers to open with xine (default, from mime settings) - plays
ogv - plays in firefox as before
wmv - offers to open with xine (default) but then fails, claiming it is missing
avi - offers to open with parole (default), but fails to initialise backend in
gstreamer
mov - offers to open in xine (default), continues to report my own are corrupt, but plays downloads
And for several of these (NOT mp4) the sound is silent.