airfoil will let you do this. But you will have to be sending audio from a computer. You also will not be able to control individual volumes remotely from an iPhone/iTouch, but you can still adjust the "master" volume
Airfoil will let you send audio
from any
mac or windows machine
to mac, windows, linux, tv, iphone, ipod touch or airport express.
The sync isn't quite as tight as with iTunes/airtunes, but pretty good. I occasionally have to disconnect/reconnect to get everything back into sync, the biggest offender is the iPhone.
this also gives you the option to send audio from pandora or last.fm.
Video is a bit trickier because there is a delay, bu there is a video player that comes with airfoil.
check out
this screen shot to see the individual volume controls.