Here’s an interesting data point from the OP. System voices are working correctly on my Mac mini after updating to Monterey, except for the right-click context menu Speech - Start Speaking. By which I mean that I can select and download any voice including Siri voices, and they work as system voice for announcements, time, etc and also for the keyboard shortcut for Accessibility’s Speak text.Apparently it's a lost cause huh?
Also odd, on an M1 MacBook Pro also upgraded to Monterey, voice selection works for the right-click Speech menu too.