Drum sounds not working

This may just be me being thick, but I can't get the sounds for any of the drums to work on my new computer. I'm running Vista and using V0.9.5, and the other instruments all play as 'Brightpiano' but the percussion doesn't play at all.
I've had a look at the mixer (F10) and that seems fine- nothing's muted or solo, but I just can't seem to get any of it to work.
Thanks!
X.