Using an mp4 or webM as the alert's image to have the video and audio matched/synced together is a great idea! ...when the video doesn't freeze during the play.

Using a video file for an alert doesn't seem to be working properly and I am almost certain it's not a broadcaster software issue because when paired with entry/exit animations, those execute smoothly, only the media's video play appears to be broken, while the audio plays just fine ? It doesn't seem to be doing that in preview but when tested, it definitely craps out on us.
EDIT2 : Apparently there are also occurences of the video not even appearing with the alert ! the audio is still playing but no picture, only the alert's text

I tried with mp4 and webM files, it's not a quarter MB big which is as much as an emote !

EDIT1 : The preview window for the alerts in the editor works fine ! the video fully plays, as expected, without echoed/doubled audio or anything... Why can't this (whatever makes up the editor's preview) be the source for the alert ? It seems to be running well without any hiccups, unlike the actual alert player used in browser sources...

EDIT3 : As a sanity check, I took the redeem's URL and put it in Firefox and Chrome, fired a Test alert, nope, video freezes/doesn't show and audio echoes, the exact same way it gets played through OBS too...