You can add this to the list of allowed file types for the images in this file:
HTMLEditor/editor/filemanager/connectors/php/config.php
Around line 142 there is the list of allowed extensions:
$Config['AllowedExtensions']['Image'] = array('bmp','gif','jpeg','jpg','png') ;
You can just add mp3 to the list like this:
$Config['AllowedExtensions']['Image'] = array('bmp','gif','jpeg','jpg','png', 'mp3') ;