I've installed XBMC on a laptop, and its working fine.  But tonight when I went to connect it to my TV and AV receiver (Pioneer VSX-922) via HDMI, it wouldn't output sound over HDMI to the receiver.  After a lot of faffing about, I managed to get the laptop to output sound to the receiver, but for some bizarre reason, XBMC continues to output sound to the laptop's speakers.  When I go into the Sound Mixer on the laptop, I can choose between the different playback devices - for the VSX-922, it shows volume control for system sounds, but for the laptop's speakers, it's showing volume control for XBMC.  The sound mixer won't let me "assign" the XBMC sound to the VSX-922. 

This is an intensely frustrating problem (and one I am not equipped to dealing with at 9.30pm on a Saturday night).  Does anyone know why it's doing this, or better still, how I can fix it?