I tried this procedure but it did not solve the problem. Moreover, I played more games on my computer and it seems that I also get the same problem from time to time. I noticed during a blitz game that I heard the countdown only once or twice although I did wait on purpose for the counter to go below 10 seconds.
The problem does not come from the iPad only. It seems more general. And it also seems that I am not the only one since I found this post Countdown voice on mobile