Assume that I commute to my work every day and if there is no traffic jam, I know what is the best/fastest route and for that I don't need any vocal instruction (to remind me, where to turn left...) In this situation it's only annoying.

The suggestion would be: add an option for "professional commuters" and for the normal fastest way, by default activate "Alerts only" and in case, there is a faster route, then activate "Sound on".