Wireless Whole-House Audio
Wireless audio is indeed a viable option for you. Generally speaking, you connect an RF (radio-frequency) transmitter to the line output of the main music system and an RF receiver to the remote system, which must include a power amp for its speakers. Alternatively, the speakers can house their own individual RF receivers and power amps, in which case they'll have to be plugged into an AC wall outlet themselves as shown above.
Most such wireless-audio systems can include several RF receivers for multiple pairs of speakers, though they all have a limited operational range that depends on several factors, such as how many walls are between the transmitter and receiver. In addition, most such systems can also send audio from a computer by connecting the transmitter via USB.
In most cases, whole-house wireless-audio systems do not include speakers, which must be bought separately. Bose does make such a system that consists of a transmitter and receiver for $400; up to six more receivers can be added for $150 each. However, as far as I can tell, it's compatible only with the Bose Lifestyle system.
The WA-50 from Atlantic Technology includes an RF transmitter and receiver for $200, and extra receivers are $90 each. Up to three transmitters can be used to send independent signals to different zones, and each transmitter can send to multiple receivers.
Aperion makes a similar system called Home Audio Link (HAL), with a transmitter and receiver for $150; extra receivers are $55. However, according to Aperion's website, the HAL system is currently unavailable until HAL 2 is introduced in August.
Also from Aperion is the Zona wireless speaker system (pictured above), which includes an RF transmitter and two speakers, each with built-in a receiver and power amp, for $500. The Zonas are intended as surround speakers, but I'm sure they would do fine in your application.
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