Welcome to the forum.
It could be a codec problem, emphasis on the 'could'. Photodex usual reply to this would be to install the K-lite codec pack which solves many similar problems.
I have only done a quick search of this forum to find a similar problem, but in this case it was the reverse, video and not sound that was missing. viewtopic.php?f=6&t=10907&p=88720&hilit=audio#p88720
. The K-lite codec pack cleared this problem.
By the way, feel this should have been in 'Trouble Shooting', not in 'Feature Requests' for future editions of the program.