Replaced german.zip with estonian.zip and at last got also the voices to work. Mentioned, that the previous voice directory had *.lua files instead of *.xml - so that was probably the reason of previous failure.
Tested it in real driving and the results are a bit confusing. Some of the directions sounded in the right place but some were rather confusing or even wrong at all...
What could be the reason?
And many thanks for "hacking" to add greater value to the Ovi Maps application in Nokia N900