Retrofitting ceiling speakers to a room is very difficult without chasing walls and inviting the wife to choose a new decorating scheme and yet many people would like the covenience of discrete ceiling speakers that can be controlled from an MP3 player while lounging on the sofa.
First the speakers:
single unit stereo powered speakers are not uncommon but, in addition, these would have an LED lighting ring around them so that they can replace existing light fittings. This would also provide the power. The lights would need to have an RF controlled dimming and off capability so that the light switch can be left on continuously to power the speakers. The speakers would also have either an RF or Bluetooth receiving capability.
The transmitter: Again these are not uncommon and would be an RF or Bluetooth transmitter that plugs into the headphone socket of the MP3 player and transmits the output to the ceiling speakers.
There would be no limit beyond practical as to how many speakers could be installed. The transmission could also be to just one speaker with the first speaker acting as a relay to the other speakers either wireless or accross the electric wiring.
Two things: this solution will not satisfy audiophiles but for the other 99.9% of the population reproduction will be dependent on the quality of the speakers used and comparable to any of the media streaming systems on sale. Most of this can already be done by buying all of the individual components but there are no complete systems on sale.