When implementing the islandora:audioCModel it failed because the default thumbnail called from plugins/QtFormBuilder.php wasn't present in the /images directory.
Here is the line from the plugins/QtFormBuilder.php file that calls for the qtThumb.jpg
$createdFile = drupal_get_path('module', 'Fedora_Repository').'/images/qtThumb.jpg';
The thumbnail should be pushed up to the github (https://github.com/Islandora/islandora/tree/master/images) and the image listed as a requirement when installing the Audio / Video solution pack. I've attached the audio thumb that we are using for a current project.
As a suggestion ... perhaps we could associate the mime type with a default icon should the image requested not be present ... so from the Crystal set
for mimetypes audio/
for mimetypes video/