Very strange.
Has turn-by-turn ever worked or did it suddenly stop working? If it never worked, might be worth taking it up with the supplier under warranty.
Alternively, it might be that some of the sound files are missing or maybe just there isn't enough space or memory available to use them.
You could try uninstalling updates on Maps (unfortunately, without rooting you can't uninstall it) then update to the latest version again.