Why are swiffy files without sound on iPads?
HTML audio has shaky compatibility across all the different browsers.

For this to work correctly, the audio would need to be converted into MP3, OGG and WAV formats. (in addition to converting the video, which different browsers prefer different formats, too)

In short, HTML audio and video is young, and a fuller experience will come once the standards mature, and the browsers homogenise format compatibility.