Muti-room airplay audio no longer working

Muti-room airplay quit working from my iMac around the time Apple enabled Hi Res and Spatial audio. I have 2 HomePods in different rooms (non paired) and 2 Airport express receivers connected to amplifier/speaker arrangements. All devices are running the latest revisions. This environment did not come about immediately following updates but some time later when the new enhancements were put into effect! All my airplay2 devices appear upon clicking on the Airplay icon but upon selecting the additional device, no sound plays through any speakers even though the time is moving on the playback until I select the computer speakers only again. Please help! Extremely puzzled?