bought an appletv last night for this same purpose.
so i have my home stereo connected through my airport express. i can stream music (audio only) from my iphone and appletv to my speakers.
and when watching tv or dvds, i have them play sound thru my home stereo, not the tv itself.
but when i watch video on appletv (say, thru the movie trailers app), it will not play the sound thru the airport express speakers.
just got off the phone with apple support and they told me its not possible to watch video and have audio thru airplay speakers. so appletv is basically worthless, since i can already airplay my music from my imac.
super disappointed in appletv.
ok, so i have figured out a work-around to use the appleTV in the way that IT SHOULD WORK.
so my airport express was being used to broadcast my internet wirelessly and for airplay thru my home stereo. first, i retired my airport express and am using the wireless setting on my modem for internet. then i had to purchase an optical cable and an optical-to-rca converter.
so now my appletv functions as the airplay device and is hooked up to my home setero via the optical/rca converter (appletv > optical wire > toslink to rca converter > rca inputs in stereo receiver).
so now when i stream from my iphone or imac to my tv, i can get sound through my home stereo set-up.