An A/V receiver has plenty of power to run multiple speakers as long as the load presented to it isn't so much that it kills it. So, if you have impedence matching volume controls of decent quality, then you can just tie the speaker wires together. Literally wirenut them together (cheap) or use something like this:
SPW-8 Eight Zone Speaker Distribution Panel
There are cheaper/similar products out there, but really they amount to wire nutting the four wires together for each channel and putting them on positive/negative for both left and right.
I actually did this for a client and it worked suprisingly well.
Change that - it was terrible, but then I took the volume controls out of the wall and found out that it had been miswired on one volume control and once I fixed the wiring behind the volume control in the wall it worked fine. Same type of cheap A/V receiver and going out (in this case) to 3 pairs of speakers which were using impedence matching volume controls.
In a setup with up to 3 pairs of speakers, you can use a 7.1 receiver and hook up 1 pair to the fronts, 1 pair to the surrounds, and 1 pair to the surround back speakers, then set the A/V receiver to 7 channel stereo surround mode. It will take a 2.0 channel source and route the stereo equally to all the left/right channels and play back with equal power distribution across the channels. That's a really good option as well, but costs a bit more. If the A/V receiver is 4 ohm stable, then you could potentially drive 6 pairs of speakers without a problem from that same A/V receiver without need for impedence matching at all.