I did make it play one pre-uploaded track, then I started to think about all the improvements I could make, like reading an SD card, adding a display with a menu, control buttons, a web interface to drag-and-drop and play tracks, etc. Basically, analysis paralysis. I'm still learning how not to do that to myself.
Here it is, biding its time :) https://imgur.com/a/vvHpyS8