For a first version, I used a easily available MP3 playerboard:

• MP3 Player: https://fr.aliexpress.com/item/1005002420437473.html
• this is so cheap and working so well, it'is incredible...

This board is connecte to a power bank:

• 18650 power bank: https://fr.aliexpress.com/item/32969695165.html

and a small 2x3W 5V amplifier:

• Amplifier: https://fr.aliexpress.com/item/1005004226201772.html

The Amplifier board is connected to 2 loud speakers (2 because stereo is COOOOL):

• loud speakers: https://fr.aliexpress.com/item/1005004832493500.html

and some nice buttons: 

• buttons: https://fr.aliexpress.com/item/1005001683893211.html

The wiring diagram is straight forward:

The MP3 player is connected to the power bank. The sound output of the MP3 player is connected to the input of the amplifier. The USB plug / buttons are deported to the faces of the box, also the USB-C plug to recharge the box.

And I printed :

• a nice box designed in Fusion 360
• a cartridge reader slot
• some cartridges containing a USB stick

in addition, to connect the USB cartridges to a computer or to a CD / external MP3 palyer accepting USB sticks, I designed slot with the USB extension cord.