It's possible to solder a wire to a GPIO pin of a Raspberry Pi and use a program to broadcast FM from that. I've done this very thing in the past. I did it with a first gen RPi, but I think the actual code to make this work with other RPi models may have changed. Google is your friend for this.

For me the performance was quite good. I could pick up the music anywhere within ~100 ft from a short 6cm wire soldered to a GPIO pin.