When trying to play .avi files on a citrix client I am getting "Windows media player cannot play the file because there is a problem with your sound device"

Now, this is only happening when Im trying to play videos using our HP t5735 clients - other audio/video plays fine (such as youtube).

Strangely, when I play the same file at home using web interface, the avi files play fine. So it would suggest something amiss on the HP clients - but what?? All the settings seem OK and I am running the latest HP image

Anyone come across anything similar?