Probably you are missing a codec. A good idea could be to download GSPOT (free on Internet) and let this program examine the PC and the video file. It tells you which codecs are installed, and which you need for the spesific video file.
Thanks asoeli. Came back next morning to try again and lo, there was sound! The Magix fairy must have been during the night. Will use your info next time I get a similar problem.