I’m aware Adafruit has an audio shield that I could plug into an Arduino Uno, but that requires assembly so is no good for my needs. And MP3 shields cannot loop sounds seamlessly or transition seamlessly from one track to the next.
Gravitech has a Nano MP3 shield but that has the same problems with looping sounds as all the rest. Still for some purposes a board like that might be okay, so if you know of any boards similar to that that would be good. I know Adafruit has one tiny Mp3 board but that’s been out of stock for a while.
The smaller and cheaper the board is, the better.