How to make a radio transmitter from a smartphone?

Question

Audio broadcast

Tell me, by what means can I implement the following idea: transmitting an audio signal from a smartphone to an earpiece.
Those. I say something into the phone, and the other person (maximum distance of 30 meters, open space, field) hears my speech through the earpiece (only hears, does not transmit the answer).

Answer 1

To do this, you need to understand the firmware of the processor that controls the radio path. Smartphone manufacturers do not advertise this information.
Plus, using a radio transmitting device in this way at frequencies that are not intended for this is an administrative offense.
Your option is radio stations.