We did some more checking, and it looks like while Safari supports mp3 audio files, for some reason it does not support mp3 audio in mp4 videos. Firefox does not internally support this, but will use a third-party decoder if available, which is probably why it also does not work on OS X. On iPad, all browsers, including Firefox and Chrome, actually use Apple’s browser internally, so that’s why no browsers will support it there. Here’s a link from Mozilla that has more detail:
So it looks like what we’ll need to do is to convert the video files after we download them to use a more compatible audio format. Thanks again for bringing this to our attention! We’ll work on getting an updated version of the channel that no longer uses the mp3 format for the audio!