A few cross-browser issues have come up during review lately:

1. .ogg files aren't supported natively in Safari, so for these files the audio player displays but does nothing, which is confusing
2. At least some videos don't work in Safari:

3. audio and video elements aren't working in IE11:

Things are looking good in recent versions of Chrome, Firefox, and Edge. We should also test this in mobile Safari, at least.

Thanks to @Etonkovidova for the screenshots and for identifying these issues!