How do I make an old Airport Express be a purely Airplay device? I have an old Airport Express (copyright information says 2008) that I have set up as purely an Airplay slave: my main router (not Apple) runs network, IP, DNS, etc, and the Airport connects via an ethernet cable to the router, allowing me to hook some speakers to the network via Airplay.

Unfortunately, I have forgotten the password to the device.  I want to reset it so I can apply system updates and the like, but I don't remember how I set up the device in the first place, and am loathe to touch it until I know I can replicate its current configuration.

What do I need to do?
The reset button can be pressed to temporarily reset the password of the device to the default. At that point, you would log in and put the device in bridge mode, update the password, turn of WiFi if you wish.

I've used these devices as you mention many times and it works well. The only catch is sometimes the latest AirPort utility won't manage older devices and you need an older configuration tool.