Here is the trick : when muting one of the soundtracks for an export in Womble, do not select the Mute option from the side of the timeline (the Mike button or the Violin button), as the mute is only operational for replaying while editing.
In order to mute a track for an export, you have to do a right-click on the track in the timeline, then select Audio, then Mute.
Thanks again for your patience and precious advice ADM, maybe you want to edit the page 1 of your guide to add this tidbit of info.
I'm now onto DVD Lab Pro. I somehow managed to get a very crude DVD last time around but I might ask you for more help along the way.
In any case, a million thanks !