I ran into this problem once before, but back then it seemed to resolve itself simply by restarting Miro. This time I wasn’t so lucky, so I had to find another solution. It was under the Tips and tricks section on Miro developers’ site and was linked to by bdelacey over at Miro forums.
The solution is to add a list key (of type string), /apps/miro/renderers with gconf-editor, and then add a gstrenderer item under it. Apparently what this does is use the gstreamer renderer instead of Miro’s default of xine, and since the videos are shown correctly in Totem (which also uses gstreamer) they’re now okay in Miro as well.
The only thing I’m left wondering is how did this solution come to effect by itself the last time, as I certainly didn’t do anything with gconf back then.