Done some more studying on this, and think this is the way:

My face has no morph targets at all (it is imported).
So I have to create morph targets for speaking.
Because I have no viseme file for this, I have to create one.
So for an "A" I make a mouth morph in shape "A". Same for the other letters.
Which letters I need, can I copy from one of the existing viseme files.

Is this the way to go?

Leo