Suddenly sound has disappeared from track four (have tried other tracks to no avail). I have tried everything but have obviously missed something. Please see jpg of audiomixer http://tv-moen.dk/audiomixer.jpg and http://tv-moen.dk/timeline.jpg
Your quick response is appreciated. It now appears that my system soundsettings were at fault and there was nothing wrong with the exported material. I do apologize for making this topic but in my frustration I could not see the forest for trees. This topic is therefore best deleted.
PS your observation on 1:31:11 led me to investigate from scratch. Thank you.