Just found the Chicago Film Archive, which is working on preserving all of Ruth Page's films (and there are many!), and one of the 63 that are on line so far is this rehearsal of La Sylphide with Lynn Seymour and Peter Schaufuss and the Chicago Ballet from 1978. There is sound but it seems quite low and it also appears that the video only plays with Google Chrome, at least so far. There is also a list of what is on line so far from the collection; some are not her ballets but just people that she liked, like Nureyev, in other things.
It doesn't say who they are but I saw the performance and rehearsal