In the United States, without a license, on FM you can operate with 250 microvolts per meter, measured at a distance of 3 meters. That comes out to about 11 nanowatts. Depending on your receiver, that's probably about 200 feet.
On AM, with some exceptions, you can operate with 1/10th of a watt with a 3 meter antenna (a/k/a an inefficient antenna). Depending on ground conductivity, frequency used, local electricity interference floor and the receiver, that can get you out a few hundred feet up to about 1.5 miles (in the most ideal conditions).
Bluetooth Auracast is the way to go with terrestrial microbroadcasting in my opinion.