I use old Android phones as players (for audiobooks only, but I guess the idea still stands for music, too, only choose a different app). In flight mode and without any SIM card, the battery of a mobile can last for weeks in standby, or for a few days of non stop audio playing.
The app I am using for audiobooks is called Voice. It remembers where you left, it can play at faster speed without altering the narrator's voice, it has a timer for falling asleep while listening to a book, etc. The physical volume buttons of the phone act as playback volume, directly, without the need of waking up/unlocking the phone screen first. Can play mp3 and most other formats, not only audiobook formats.
The audiobook playing app is free and open source, no ads, called Voice:
https://github.com/PaulWoitaschek/Voice or can be installed directly from Google Play, F-droid, etc.