The device has no output controls

I've been using the original model of JBL Creature speakers for many years on a series of Macs. Today I tried plugging them into my Airport Express to set them up as AirPlay speakers that I could use with my Mac and iPad. I transferred the speaker cord from the Mac to the Airport Express, opened the Sound control panel and selected Airport Express as the output device, but I got a message stating "The selected device has no output controls." Enable AirPlay was checked in AirPort Utility, and all my software was up to date. I reconnected the speakers to my Mac, chose Line Out, and they started working again.

2010 MacPro

Airport Express version 7.6.3