use an alternative format when mp3 is not supported

chromium doesn't decode mp3 (licensing reasons) -- it would be nice to specify a fallback there (e.g. an ogg).
However, chromium doesn't seem to return a no to
document.createElement('audio').canPlayType('audio/mpeg')
which makes detecting this hard. Maybe trying ogg first after checking document.createElement('audio').canPlayType('audio/ogg;codecs=vorbis') returns 'probably' is a better way.