Siri voices on macOS Big Sur

Hi all,
Apologies for crossposting (if someone has already covered this topic before, though I could not find any information on it).
I am using MBP 2017, with MacOs 11.2.3 (the latest software update from Apple).
The issue I have been facing since the last OS is that my British and American Siri voices (which I tend to use as my VO voices) are failing to change their speaking rate. . Despite my attempts to change rate, they remain obstinately slow.
I performed number of troubleshooting tasks:
1. I deleted my Siri voices from the system directory and reinstalled them many times, no joy!
2. I removed Infovox-Ivox and supplementary softwares thinking that it might be causing this issue and then removed the Siri voices and reinstalled them again, no joy!
3. I repaired the OS using the recovery mode, no joy!
The only thing left for me is to delete the OS and install it afresh, which, needless to say I would not want to do unless absolutely necessary as I am currently finishing my PhD thesis and would prefer to keep any software related interruptions at the minimum.
The Siri voices do help me concentrate on my work, particularly the British male voice.
Can anyone suggest anything to help me out with this?
Thank you and have a good day!