Cannot Adjust Siri Voice Speed on Big Sur

Good day,

I am unable to adjust the Siri text-to-speech speed in Big Sur. I go into system preferences, then accessibility, then spoken content. I've checked speak selection. I've downloaded all the various English Siri voices. I drag the slider to fast, and the voice reads at the same speed as the default setting. I pull the slider to slow, and the voice reads at the same rate as the default setting. This issue happens with all the English Siri voices (and possibly others, but I have not tried them. The non-Siri voices do not seem to have this issue. I have uninstalled the voice packs and redownloaded them to no avail. The Indian English Siri voice had this issue on Catalina, but it appears it affects all of them in Big Sur. I found one post on another site with the same issue (https://www.applevis.com/forum/macos-mac-apps/siri-voices-macos-big-sur), but they have no replies, so I know I'm not the only one with the problem. With online schooling, it has been helpful to have a built-in text-to-speech reader. But now, it appears to be bugged. Any help is appreciated.

Thanks!