Looks like the 2009 Wikimania presentation videos on Commons are buggy (I think they were chopped from streams in a weird way) and don't play in Chrome... or correctly transcode to WebM.

Found an easy way to rewrite the file structure with ffmpeg while keeping the original audio/video encoding intact (-c copy); oggz-chop will also add back a skeleton metadata track which ffmpeg doesn't know how to create. Notes at https://commons.wikimedia.org/wiki/File_talk:200908281410-Andrew_Lih-Challenges_of_Covering_the_Wiki...

Now I just have to automate the download/fix/reupload process on the other 82 presentations from that year... sure as hell not gonna do all those files by hand. ;)