Audio problem: WASAPI static noise & DirectSound dubbed sound

Hi,

I have an Asrock ION 3D htpc running Win7 64-bits with XBMC 12.1. My htpc is connected to AVR Marantz NR1403 with HDMI. The sound in xbmc 12.1 frodo is bad. Using WASAPI Nvidea Marantz HDMI, the sound seems okay, but I get frequent static noises (every 5 seconds you hear a distortion).
When setting xbmc to Directsound HDMI Nivdea, the static noises are gone, but the audio appears dubbed (wavy sounds rather than constant). For example the constant sound of a train would sound like a wave instead of a constant sound.
I followed all sections in this topic: http://forum.xbmc.org/showthread.php?tid=146911 and installed the latest Realtek drivers. Also tried uninstalling Realtek. No improvement on audio. Video is working fine.

Setup:
XBMC PC -> AVR -> TV
All using HDMI cables

Some specs:
- installed all Asrock ION 3D drivers (nvidea, realtek, ...)
- the option "use marantz exclusive for sound" is active in sound settings (windows control panel)
- No audio problems with open vlc and Plex running the same movie.
- HDMI CEC is disabled
- Setting audio to Analog or Digital didn't help

I have uninstalled xbmc 12.1 and installed xbmc 11.0 eden. Similar problem. The guisettings.xml all have "1.000000" as value for audio volume settings. I didn't change that.

A friend of mine is using asrock ion 3d with win7 + xbmc eden and onkyo receiver without a problem.

Any suggestions on how to fix this? Thanks!