It is possible, but someone would have to design software that sent the audio out to all the sources and kept it in sync.
Keeping it in sync would be the hard bit - transmitting to wireless speakers and having the MBP speakers play exact in time would be tricky. Check the video delay out when you use Airtunes to stream audio from a film or something.
Only way I got round to getting music playing in different rooms at a cheap way is to get someone to stand in one room and me in the other, and count 3,2,1 "press the play button!"