AC3 ATI HDMI Output problem

I've just upgraded from vista to win7 and i am runing in troubles trying to play mkv files.
After intalled mkv divx preview, in order to enable mkv support on win7, i can play then via WMC and WMP but without sound.
I have sound using vlc.
then i tryed Media Player Classic.
No sound too.
But i've managed to listen the sound output from mkv files using MPC by changing de DirectShow Audio divice to ATI HDMI Output.
The problem seems to be the mkv's AC3 track
Using vista i have no problem listening AC3 tracks

any help please!

thank you