Thank you. There's definitely something wrong with the files as I can't even open them in macOS. If you hit the button menu on the Playdate, and then go to the "Log" screen, you should see messages like this:
> Cannot open audio at '/mp3/Birthday Game Ep. 5 6 - Bee and PuppyCat - Cartoon Hangover.mp3 as a sampleplayer: [wrong file type: no header]
Reencoding your files with ffmpeg (with just `ffmpeg -i yourfile.mp3 myfile.mp3`) worked for me. Hope this helps.