So, tested it in a real life yesterday. Calculated the route indoors with wifi connection, then went to the car, pressed the "red button at the right upper part of the screen" and started to drive. In the first test the voice sounded once and in uncorrect place - then soon I mentioned that the screen went black - the program has exited the tracking mode. Pressing again the red button returned it to that mode, but then there were already no voices... In the second test I got the voice commands twice and in correct places but then again the screen soon turned black, and thereafter no voices...

Well, it seems to me, that Nokia has not included the voice guided navigation in Ovi Maps not just to be despiteful, but for a reason - combination of N900+Maemo is not enough powerfull to allow problemless running it whilst symbian is just a much less ressource hungry op. sustem...