AirPort Express will not output music to Home Stereo

I bought two used AirPort Expresses. I've set them up and both have the solid green light but neither of them will output music to my home stereo. They've each set up a wi-fi network. But no matter what I do I cannot get my iPhone SE or iPad to play music through the AirPort Expresses to my home stereo. When I play music on the devices it just comes out over the small speakers inside the device. The AirPort Utility app on my home iMac and both my devices recognize the AirPort base stations and show green lights as their status.

I have the base stations hooked up to the stereo via the 3.5 mm jack that I've successfully used from my iMac for years. I have an ethernet connected to the base stations from my wi-fi router.

This is my first attempt to use AirPort Expresses.

I must be missing something! What could it be?