UPDATE 2006.02.27: In iTunes 6.0.3, Apple seems to have put a band-aid on the problem by making all the speaker outputs obey the main iTunes volume. Once again, that just lowers the volume everywhere, including on the computer. It’s not a perfect solution, but I’ll accept it for now. 
Thanks, Apple!
Nearly unnoticed in the recent iTunes 6 upgrade has been the enabling of multiple AirTunes speakers, apparently synchronized via ping times from router to router. This is a feature I’ve been hoping for/expecting for some time, the next iteration of which ought to be in DVD Player, syncing streamed sound with DVD video (the VLC team should be implementing this).
It’s really cool to be able to sit on the couch with the stereo behind me (which has less-than-stellar tweeters), and my computer in front of me (without any bass whatsoever), together playing full sound, in perfect sync. My only gripe is that whenever I use multiple outputs like this, my remote speakers are always set to the highest volume possible, with no apparent way of changing it. The iTunes volume slider controls the computer’s speakers, but has no effect on the remote ones. All I get when choosing multiple speaker outputs is a window like this:
The Rogue Amoeba folks (who also implemented syncing of remote speakers - I’m not sure if they did it before Apple, or if Apple included library support for it first) have got the right idea. They include a little volume slider for each speaker (only one shown in my screenshot, since I don’t yet have more than one Airport Express; another screenshot is on the AirFoil site):
I propose that Apple do something similar. I’ve even mocked it up for them below. It doesn’t seem like it would be that hard, since they already have a list with widgets in the cells. I know it would be way easier with Cocoa Bindings, but iTunes isn’t built with Cocoa, and probably never will be.
Right now, I’m successfully not deafening the neighbors by lowering the pre-gain on the iTunes equalizer, but that just lowers the volume everywhere, including the computer speakers. Yes, I know I could go over and change the stereo volume, but not really, because it’s an old one with a lid (see lower-right of this picture) and it’s behind the Christmas tree at the moment. Besides, I shouldn’t have to, because it should be built-in. And I don’t feel like paying for Airfoil to fix a feature that Apple broke.
I’m wanting to make some funny joke about “less than stellar tweeters.” But I can’t think of any.
I’m glad you posted this because I’m in the middle of a decion process right now to set up an Airport Express network in my house. Here’s my dilemma. The G5 with all of my music sits in my computer room upstairs. My DVD player in the living room doesn’t seem to want to play audio CDs and since all of my music is on my computer I want to find a way to get it to the living room.
I thought about buying an iPod (I still don’t have one) but an iPod would cost too much to hold ALL of my music and I don’t venture far from home to need one. So I’ve been considering getting an Airport Extreme card (which our friend Andrew just happens to have one laying around) and the Airport Express. This way I can plug in the Airport Express in the living room, hook it up to my recently purchased Harmon Kardon AVR-120 receiver, and listen to my entire iTunes library through my Polk bookshelf speakers in the living room.
On a related note. I think that the volume from iTunes probably gets streamed at full volume for several reasons. I’ve noticed that when using the FM adapter for an iPod in a car that if you don’t turn up the volume on the iPod, the audio coming through the speakers is less than perfect. Maybe getting as much volume as possible to the end device and letting that device handle the final power output is a good decision. I’m sure someone else has much more detailed input than I do on this matter though.