No, cant be that, because when I open BiaB, all instruments are basically "on", and I only mute some for a specific operation within my work... (OK, I have checked it anyway, not to look too stupid afterwards!)... Buuut You, john took me to the solution (and that is the nice thing in this forum, exchange brings often a solution).

I have opened the sequencer button (sequencer/sequencer on melody) and see, all instruments from the midi file are there all right... Only all unchecked!
Checking them all brings life back in my drawing room: works!

In a second run (a new loaded midi file came again with no sound with all instruments unchecked again) I checked the box "load/save channel data" and everything works now as it should.
Thanks to all for the advice sent to me before I would go nuts. file closed!
Bernard