It seems that if the movie is hosted on a shared repo, the player is unable to find the 'alternative' versions of media and does not present the source selector. Sort of critical in that most movies will probably be hosted on the shared repo :D
This clip has 2 webm resolutions, but on en.wp it can only shows the 360P version.
Version: master
Severity: normal
URL: https://en.wikipedia.org/wiki/File:Sintel_webm_extract.webm