Add selects to the audio file info form to make nodes easier to edit

if getid3 isn't installed the user needs to enter the file info manually. currently it's just text boxes that don't make it clear what allowed values are. the attached patch (which i'm committing to HEAD and DRUPAL-5) uses select boxes to the fileformat, sample_rate and channel_mode fields and adds a description to the playtime field explaining it's format.