I'm on macOS 11.2 using a 16 inch 2019 MacBook Pro. I noticed when I use the text to speech functionality (system preferences > Accessibility > spoken content), the Siri voice doesn't adjust it's speech rate when using the slider (or the button on the popup controller). One thing I've noticed is that any other non-siri voice works. Furthermore, the voice was working till I reimaged my computer. Lastly, I have an older mid-2015 mac and it works just fine (even after I completely wiped it).
I tried to copy over some plist files from my old mac to see if I could get that to work, but that failed.
Any suggestions on how to fix the broken functionality?