Does it have optical out?
Sounds useful. But given that a lot of modern equipment has optical in, it would seem a bit daft to be restricted to analogue only.
The optical out on an Airport Express is really handy for feeding in to a "proper" DAC (mostly because the onboard DAC one is pretty poor).
Also (probably not of major importance), is the audio stream encrypted at all or is it broadcast in the clear? I hope they're using something a bit more robust than the "wireless webcam" 2.4GHz which lets you snoop on your neighbours with ease...