If for whatever reason you actually use the files app on your phone/ipad to store mp3 files, first off, you’re an idiot, but secondly, this shortcut will play any mp3 files in a folder as long as there’s nothing else in the folder. It does so randomly and for a specific amount of time. Enjoy!
Added a prompt to ask how long to run the shortcut for. Now you can run the shortcut for as long as you want.