If you can get a normal radio to work, you could listen to that through your fm ear muffs. Buy a little cheapy fm transmitter (like ebay number 401226289894) plug that into anything that plays music, via a headphone socket. In your case a radio, or computer, or phone. Set the transmitter so it transmits at an unused frequency- I've found 108fm doesn't interfer with anything, then tune your earmuffs to that.
It'll boost a good fm signal, perhaps in your house and transmit that probably up to a 50 metre radius. If you search long enough you should find one that can be powered by 5v usb. The one I linked to does it, just I haven't used that particular model. I've used one that looks exactly like ebay number 291120418489 and is powered by 5vusb. I've used a version of that for years and it works really well. It transmits a good signal from the poorly insulated side of the workshop, to the new extension, which is like a faraday cage.
Cheers
Coley