Recently I found out that I must use headphones or disable silent mode on iPhone to listen to ogg files on Moodle (MDL-65943). I then realized that this is an upstream issue with ogv.js. This issue can be reproduced by visiting the ogv.js demo page: https://brionv.com/misc/ogv.js/demo/

1. visit the demo page with an iPhone
2. enable silent mode
3. start the video, there's no sound even if you turn the media volume to max
4. disable silent mode, immediately you can hear the sound.

For your reference, I'm using iOS 14.8

I'm created an issue on upstream as well: https://github.com/brion/ogv.js/issues/600