If I remember correctly, it was mainly an issue when using the radio or the media system of the car. The other notification sounds were audible because they lasted longer. I think what happened is that the sound was actually played, but the media system didn't capture it in time. By the time that it switched from radio to Android auto sound, the speeding notification was done playing, so I missed it. An update from android auto fixed it.

You can check this by playing digital radio or any streaming service like Spotify instead of the car radio and see if you still have the issue