Is it mandatory to upload a thumbnail for each audio file?

While it is recommended to upload a thumbnail along with the audio file upload, it is not mandatory of course. You can just upload your audio files and add thumbnail graphics later. A default thumbnail will be added automatically to your uploaded files until you uploaded your own thumbnail.

Please note, that while thumbnail graphics are a great way to promote a content visually, your ConnectedTV apps and all your publishing properties will work without thumbnails, too.