I was talking about the no-sound symptom.
Well, I tried installing the DivX plugin and disabling the VLC plugin. Doesn't do me any good, I still get no sound.
But here's something I don't understand: if I disable both and VLC plugin and the DivX plugin (and the Quicktime plugin, and any other plugin that lists video MIME/file types), the mkv file still plays in a player window that looks exactly the same. Chrome's plugin infrastructure seems very poorly implemented and very poorly documented - it's hard to control which plugin plays a file or even to tell which plugin is responsible for playing it. I was able to get mpeg files (and other video files masquerading as mpeg) to play by disabling the Quicktime plugin, but now I'm not sure if the VLC plugin or DivX plugins are doing anything.