This is a very strange problem.
I realized, that after SW V27 upgrade, I cannot change the nav volume in the settings menu. I can adjust the volume bar, but the loudness is still the same. The only difference is, that the background music (radio, CD, etc) is muted more, when the nav volume is set to louder value.
I put another radio module into the car, and the problem was solved, I could adjust the volume. After that I installed the V28, and the problem was back again. Then I was really nervous, I put my mk4 into my friends car (he can adjust the volume), and with my mk4 I also could change the volume. So it depends on the BM54!!!

It seems, that meanwhile upgrading the nav unit sends something to the BM54. If it is true, maybe it is programmable via gt1, inpa.

Does somebody have info or same problem?

Bombolo