[bluray]

Towards the bottom there are two settings:

Audio output device

Passthrough output device

Do you have both set to HDMI?

Both WASAPI: HDMI.
I have changed the audio output device to DIRECTSOUND: HDMI and I hear the GUI sounds now.
XBMC use the passthrough output device for the checked supported formats (Dolby digital, DTS). Am I right?
So it doesn't really matter when I set only the passthrough device to WASAPI, because xbmc use this almost every time during movie playback?

I found that Realtek HD audio condecs worked great when connecting HTPC to LEDTV via HDMI and connecting HTPC to AVR via option/coax. The GUI sound worked with WASAPI too......if you want to try it, you can download it from here- High Definition Audio Codecs....

[Bombaij]

I had to disable "output stereo to all speakers". Now I have GUI sounds with WASAPI.