function getAudioURI(base)
{
    var extension = '.mp3';

    var audiotag = document.createElement("audio");

    if ( audiotag.canPlayType &&
         audiotag.canPlayType('audio/ogg') )
    {
        extension = '.oga';
    }

    return base + extension;
}
Returns the URL of a supported audio source based on the user agent
@param {string} base - media URL without file extension
@returns {string}