About mp4 not working on AnkiMobile, it’s odd, considering it was added as [sound:filename.mp4], everything was synced and Tools > Check Media doesn’t report anything missing. Just in case, I’d probably make a new card using this bigbuckbunny.mp4 file as it should work on AnkiMobile, and if it works maybe MediaInfo could give some hints why the original video file doesn’t work on AnkiMobile.
About ogg,
According to Apple adding WebM audio codec support to Safari with iOS 15 - 9to5Mac
Safari in iOS 15 Beta 5 adds support for the HTML5 “audio” tag to play the Opus audio codec (only) inside of a standalone MP4 file (only).
I don’t have an iPhone, but here’s a sample deck for anyone who’d like to test bbb-opus.mp4 with AnkiMobile. It’s about 10MB. The front side contains the audio file added as [sound:audio.mp4] and the back side contains HTML5 Audio that is not officially supported and might not work, or might work but autoplay by default won’t work.
