>But for wireless speakers, the wireless signal does not transfert energy.

It does transfer energy is just not enough to power the amplifier or to directly drive the membrane.

RFID is an example of short-distance radio communication where the signal powers the tag. Contactless payment with credit and debit card use RFID to power chips that do encryption to make it safe.

You could have a transmission on one to power the headphones it would just be very wasteful of the phone battery. There is an audio transmission system that can and just that. You can build a crystal radio to receive AM radio and it does not need any power source.