Exporting would take again a few hours before completing; so I dragged the video to the desktop (where is that user's guide that teaches us all of these features?) and then it plays with sound - even better sound than when played from within Voilà .…
thanks, josue and support.
Now I have another problem, but possibly not Voilà -related...:
when I play the recorded screencast in Voilà , all is well, sound plays. But when I export to a QuickTime-movie, the sound has dissappeared! Not helpfull …