@Dsparil @locomotor79 I tried running the game (downloaded the Zippy mirror) and I didn't trigger either of your issues myself. I'm honestly a little stumped.
For the former, the makeVideoPlayableInline function is declared in rpg_core.js, which is one of the... well, core Javascript files that all RPG Maker MV games come with as standard. If you can see that it exists inside the games' "./www/js/" directory, it
should be working unless it's been modified or corrupted somehow, or for some reason it can't be read. If it's missing and it's also not in the archive you downloaded, maybe mention it here so someone can take a look at whether or not one or more of the mirrors are off.
For the latter, the Zippy mirror's got the audio files encrypted into MV's specific audio format (*.rpgmvo), so I'm not even sure how or why you'd need to tinker with decrypting them into their original formats at all to solve the issue. RPG Maker MV tends to not really look at the file-type so much as the file-name and gives preference and priority to its proprietary format over decrypted versions of the same content (i.e.- if you have both Music1.rpgmvo and Music1.ogg in ./www/audio/, it'll try to open Music1.rpgmvo all the time). So you'll not just need to rename the file, but you'll need to either remove, relocate, or change the original file's file name or file type (if the latter, to something non-audio and easy to identify if you wish to undo it would be ideal- like *.bak).
Can both of you check that the game's in a drive and folder that doesn't have partial or full Read Only status applied to its contents? And if it turns out they do have it applied when you check the folders' properties, can you try clearing it? It might help.
If that doesn't work and the issues continue, I've heard that for whatever reason some people have more success running MV games' "index.html" in Firefox than in Chrome, assuming you were running it in that browser. If that lets you play the games, then that'll have to do for a last-minute solution until someone more knowledgeable figures out what's going on with your specific situations.