I bought a jelly pro (miniscule phone), rooted it, added some automation to autoload a music player on boot and activate airplane mode. I added a 256gb memory card with a lot of tunes. It's now my mp3 player. When I deactivate airplane mode it scrobbles my played tracks and 15 minutes later it automatically reactivates airplane mode unless the phone is charging, in which case it's available for wireless music sync from MusicBee. It also powers itself off after 30 minutes of being idle. Best damn mp3 player I've ever had.