I’ve had the older 802.11g Apple Airport Express sitting in a box unused for a few years. Ever since Apple updated its Airport Utility management software to ignore this older model, it essentially became unusable. Until now…
Well, I guess this solution has been out there for some time, but I only stumbled upon it yesterday. You can still run the older Airport Utility v5.6.1 that can configure the older Express, but you need to use this special launcher. This “app” makes the needed changes so that v5.6.1 can launch on newer Mac OS versions including Ep Capitan 10.11.6.
Word of warning though. If your Express is previously configured, DO NOT factory reset it. This makes things much more difficult. If you do so, the Express goes into a configuration mode with a brand new, generic Wifi network. You need to connect to this temporary Wifi network in order to configure the Express, but when you attempt to do so, the newer version of Airport Utility launches, and of course, this version is incompatible. If you try to launch v5.6.1 simultaneously, it won’t connect with the Express. So if the Express was previously configured, you could connect and make any necessary changes. But if reset, you need to connect via wired Ethernet. Not hard but a hassle, especially if you don’t have all the pieces and parts handy (like a Lightning to Ethernet adapter for a MacBook Air).
I configured my old Express to simply serve as an AirPlay relay with connected speakers. Good enough use for this aged hardware. And yes, it works! I can stream music from an iPhone 7 running iOS 10 via this older Express.