How do you play music on a Marshall speaker?

Not very good, I can't play a guitar. 

Like this...

DeKay

Loudly or you don't get enough 2nd order.

If your query it to the BT Marshall speaker(s) then see if it has a hardwire input.

My limited experience with BT speakers being fed by a SmartPhone (maybe an iPod in one instance) has been that the hardwire hookup sounds a lot better than the BT hookup.

Patch cords for such are readily available.

Not certain if the new phones/Pods even have hardwire output capability.

DeKay

Musical Instrument amps and speakers are not made to faithfully reproduce music. They are made to create music. See if it has a CD or High Level input. If so, find or make a cable that will allow you to plug into your signal source and also into one of the amp's inputs. You may be disappointed with the sound. I hope not. Good luck.