About secondary audio files:
Not all browsers (most notably Firefox) support mp3 and/or aac for HTML5 audio. The secondary audio file option allows you to specify a second file that will be used for HTML5 on Firefox (assuming it's an ogg file). This is not required because the player will fall back to flash if there is not an HTML5 compatible format available.
Older versions of concrete5 (5.6.0.2 and before) do not allow ogg files to be uploaded by default. To enable ogg file uploading go to [your site]/index.php/dashboard/system/permissions/file_types/ and add ogg to the list of allowed file types.