A lot of examples demonstrate multiple source tags nested in the audio tag, as a method to overcome codec compatibility across different browsers. Something like this -
You can use the same DOM methods as with any other element:
var source= document.createElement('source');
source.type= 'audio/ogg';
source.src= 'audio/song.ogg';
audio.appendChild(source);
source= document.createElement('source');
source.type= 'audio/mpeg';
source.src= 'audio/song.mp3';
audio.appendChild(source);