Audio attachments are pretty much like any other post types, in WordPress. Therefore, even for an audio attachment in the Media Library, it is possible to set a Featured Image.
How to proceed:
- Go to your Media Library
- Click on any audio file
- In the Audio Details dialog that pops up, click on the link Edit more details
- In the post editor, you can change any details for the audio file, like you do for any other post or page: click on the “Set Featured Image” and choose the image you want to use as a thumbnail for this audio track.
After completing this process, that specific audio file will have the selected picture as a thumbnail in any instance of WavePlayer.
Alternatively, it is possible to embed the cover art picture in the MP3 file before uploading it to the Media Library. When you do so, WordPress extracts the cover art picture from the audio file, adds it to the Media Library and attaches it as a featured image to the audio file it extracted the picture from, without any further action from the user.