It's 10m
However I can play the sound ok with MPCHC+DXVA+Matroska+FFaudio so I don't think the issue is with my HDMI cable.
Anyway the same happens if I only send the audio that way but keep the picture on the monitor.
Unfortunately I can't get rid of the frame drops in MPC which makes a choppy playback this is why I was blown away by the smoothness with Spalsh.
So now I just have make the HD audio passthough working with Splash and I'm a happy camper.