Making a Broadcast Radio
Before going into detail, a disclaimer that broadcasting over FM brand is illegal without proper licensing. That being said, it is possible to broadcast audio over FM with a Raspberry Pi and that is too interesting not to talk about, especially considering how weirdly available tutorials for this are. The board uses a Python script called PirateRadio to run the ship, taking advantage of a feature the Pi has where it can lessen interference with other nearby devices using a technique called spread spectrum to mitigate impact on anything nearby, with the side effect of being able to create a smaller but longer reaching interference that can broadcast FM.
This strange and legally dubious device can be made with surprisingly simple tech as well. An antenna is as simple as soldering jumper wire to stripped wire and then covering the jumper segment of this Frankenstein of an antenna with heat shrink tubing to protect it. From there, it is as simple as putting mp3 files of your choosing in the correct folder within the SD card your Pi boots from, and modifying the configuration file for the PirateRadio script to your preferred settings to broadcast it and setting a frequency that your radio can tune into. Again, disclaimer, this is illegal to do in most countries without a license and is not condoned. If you wish to listen to music for free, Spotify is a more simple and legal option. Example below.