solution found:

seems to be an error in the guide -
there is either a Microsoft ADPCM
or an IMA ADPCM, not both in one.

analyzing one of the wav-files voice_editor
extracts using Windows Properties names the latter.

used Cool Edit Pro, resampled the Microsoft ADPCM
files into IMA ADPCM (89 kbps, 22050 Hz, mono, 16 bit)
and got the Nuvi working.

regards
Damian

ps
it really annoying - codecs, Java, C and so forth,
Micrcos's own custom (i.e. licence) variations are
often polluted to uselessness...