[eluser]Seppo[/eluser]
Try to see what mime type is you was file (echo $_FILES['your_field_name']['type']) and add it to the mimes config in application/config/mimes.php
Currently it only supports 'audio/x-wav', you should change the line to
'wav' => array('audio/x-wav', '???'),
being ??? the type that you got from printing.