@flovo Thanks for trying too
Here are the answers to your questions.
Am I right that the problem happens when you are on a mobile internet connection with poor quality? Old device (slow OS, device)?
I tried as an experiment to play from the same internet connection (which is my home connection, good enough to watch streaming videos) with the mobile device and with my computer. It appears that the problem also occurs with the computer. When playing a blitz game from the computer, I voluntarily waited to be in the last seconds of the byo-yomi. I would say that out of 10 times within the last 10 seconds, I only heard the voice 3 or 4 times. It also seems that there is a very slight delay between the clock display and the voice (the voice is heard a fraction of a second after the clock, when it is heard).
When trying again with the mobile device and the same connection, I heard only the number “eight” one single time when trying byo-yomi.
The mobile device is newer than the computer so it does not seem to be the cause.
Does the clock show the correct remaining time?
Yes, it does.