I know what an Airport Extreme is - I use two of the older "G" version of them here, as well as an Airport Snow. The two "G" models actually provide a "Wireless Distributed Services" connection. (A base station to Base Station link that works without wires to provide extended range.
The pre-"N" versions also contain a modem (almost irrelevent now), and both extreme versions act as print servers. The new one is also supposed to act a file server. (Hmm, I wonder what kinds of files you would want to serve?!?!)
The express, for me, was best used as a portable base station for travel to those places that don't have wireless access, but do have ethernet based connections to the outside. I was aware of its audio connection too, but that was not a primary driver for me to look at it.
Originally Posted by kennyt
The Extreme is a router, the Express is the receiver with the digital output and to date they are only 802.11g.... Now if they become 802.11n, I'll redo the network in the house then.