I've not tried this yet, so can't comment on whether you mean the default setting for voice commands is too quiet. However, in danger of stating the obvious, have you made sure the volume on your N95 is 100%. Failing that, why not try using one of those FM devices that outputs to your stereo. Then you could have the sound as loud as you liked (though you wouldn't be able to listen to your stero )
azzy, the first time I used it, I too couldn't work out how to get voice commands! But its there, you need to turn it on after you've set the route, just follow the options menus... might seem tricky at first but you'll get there!
The first time you use voice guidance.. You need to be connected to the Internet (better if its a WiFi) then chose a any destination to navigate .. The sofware will then start downloading the Voice commands from the internet .. you need to do this only once.. then later you can hear directions when you navigate without having to be connected to the Internet..