I have all the sounds turned off, the auto-recalculate turned off, too.
If you start to use the device as a map (and as a much simpler aid to just being on the correct road) you may well find it much easier. Use your eyes, not your ears, to tell you what the speed limits are or are likely to be. If, on say a dual carriageway, you see the vehicles up ahead applying their brakes for no apparently good reason, ponder if it might be because there is a speed camera or some other problem that will (when you arrive there, too) will require you to maybe slow down as well...... And adjust your speed accordingly BEFORE you arrive at the same spot.
As you use the device more and / or go further afield, you may well find it becomes increasingly annoying that you are told every three seconds that you are in a speed restricted area, when all you really want to be doing is concentrating on busy traffic in a complicated road system. When you get to that point in your life you will turn the sounds off and never turn them on again.
If you are running a route, as apposed to just having the device running, you will notice that the local speed limit is displayed in the bottom right hand corner. If you are running a route, you will notice that the display changes reasonably accurately with the change in signed limits. For instance, cross the boundary from a national 60 mph limit into a village and the display drops from 60 to 30. But you probably knew (as you passed a driving test at some point) that the limit in the village is probably 30 anyway. Does the device 'know' all the speed limits? No but it 'knows' most of them and can display them when it does.
In short, stop using the device as some sort of super driver, sitting on your shoulder whispering in your ear every 10 seconds. Take control of your own life, by employing some common sense. You'll know ( or at least you should) what the speed limits are likely to be and / or when you have arrived at a destination. You have noticed that the device is apparently missing warnings, so you are probably using your eyes anyway and don't need to be told. You are halfway there already, prepare to make the next step forward.