Thanks for the info and link. Good clue.
I re-encoded so that the moov atom is at the beginning of the file and have the same problem. Further, we have no problem direct playing standard Adobe encoded h.264/mp4 files on IE 11 from our website. Given that PlayCanvas plays video through an MP4 audio function, I’m still suspicious that this could be a PlayCanvas issue.
I’ll put together a dummy page and see if my files will play on IE 11 otherwise just to be sure.