Because I couldn't hear the nav. voice guidance above the radio at reasonable radio levels and far from excessive road speeds, I took my G3 along to the dealer, who agreed that the voice level was nowhere near high enough on max - indeed he had experienced an identical problem on another Prius. The aftersales manager has investigated the manuals and can find no means of tweaking the levels other than the user-accessible settings via "VOICE" on the main settings screen, which I've already set to level 7 (max). The dealer has agreed to register this with Toyota as an issue and I understand that if enough such registrations are received they may do something about it. So, if you are having the same problem and you haven't found the answer get your dealer to do the same. If enough of us complain then it might get fixed.
Chas