Nigel.
You keep on talking about MGU.
MGU is a proprietary BIAB format. This will not help you in any way to narrow things down.

Did you try to play WMA file within windows with BIAB closed??
Lets do that to determine if you need a codec (which is possible, but unlikely)

Again:
1)Close BIAB
2)Go to the drive where "bb" folder is located and to bb/Demos/RealTrack-Demos
3)Scroll down the list to WMA (Not MGU) files and try to simply play them by double clicking on them.