In System -> (Settings) Posting -> General -> Attachments, there is no option referring to Audio.
And when looking at the allowedFileExtensions function in System\Helpers\Form\Editor.php, the 'media' choice doesn't merge in \IPS\File::$audioExtensions
As such, the only way to allow someone to upload audio files is to either use 'All' or the 'Allowed file extensions'
Similar situation to when I brought up video upload added to 4.3
In addition, you don't appear to account for audioExtensions in the _addAttachmentsToContent function in System\Content\Api\GraphQL\ContentMutator.php, while accounting for videoExtensions there.