My Atak would play any MP3. It was a pain for Audiobooks because it would not use a playlist and chained the files in whatever order it found them
A Sandisk Zip+ is
Tiny. It handles some Audiobooks (encrypted) as well as MP3 playlists. I use 'On The Earphones' which, one earpiece is bigger that the Zip, but they do help when riding on a noisy BART