Sure, it could certainly be used as an MP3 player. It has that capability, to some extent, already built into it. The phone can also be used as a hand held, small screen tablet when not activated. It still works over WiFi whether it is activate for mobile phone use or not. There is a Google audio player app already on the phone. I’m sure there are a multitude of apps in the Play store that might even make it a better MP3 player.
Music or podcasts can be downloaded for offline play. Or, it can stream audio when connected to WiFi. You can even enable GPS and use it as a road navigator when traveling if you have an app, such as HERE WeGo, that lets your download maps for offline use. Google Maps also allows you to store map areas for offline use.