WMV file: audio cuts out in middle but video keeps playing

Hi,
I have a 9 minute WMV file with a problem!

After the 6th minute or so.. sound cuts out (where there should be sound)
This happens with windows media player but also with all other media players ive tested...

Does anybody know if this is a fault of wmv?

the sound cuts out.. but the video keeps playing
and i know for a fact sound should be playing.