Digital receivers which are in cellphones are most certainly based on A/D convertors which is what you originally referenced. All captured antenna energy is dis

Digital receivers which are in cellphones are most certainly based on A/D convertors which is what you originally referenced. All captured antenna energy is dissipated as heat through resistance in order to extract useful information and convert it into the digital domain. Even in a analog receiver the power amplifiers used to drive a speaker or whatever are going to dissipate the signals power as heat in order to modulate the amplifiers output power (out power will always be less than power supply power). If you are talking about a crystal radio, then yes the input power drives a speaker and is turned into sound instead (along with some power lost heat in the circuits resistance).