Since the change was on Apple’s side, have you checked with them? Sonos hasn’t changed their software (recently/yet) 

Yes. It’s a security improvement!!

I can haul files out of the hidden podcast library folder, paste them into my iTunes folder and rename them but it’s a right faff.

Heh. I’m an Apple user myself, but sometimes their ‘improvements’ cause me to shake my head. I’m sure there was probably some dastardly loophole it resolved, but it feels a bit like cutting off your nose to spite your face.