That's good.
Does that mean that I will not be able to play a DVD with PCM or DTS sound with the best quality through my Asus audio card ?
Is there a way to play Bitperfect such DVD ?
Well... It depends how you look at it. It won't be bit-perfect, in most cases, because the Windows mixer will be ... Mixing it with other audio sources on your computer, which by-necessity means it isn't bit-perfect. So it is best, theoretically, to avoid the windows default mixer if possible. However, if you use the volume control in any application to modify the volume of the output, then it also isn't bit-perfect, so that may not be a goal relevant to your needs.
But either way... The way this is done is by using an audio rendering engine
other than Direct Sound or Wave Out, usually either WASAPI or ASIO.
What do you have
Options -> Audio -> Audio Output -> Output Mode set to? Does Audio playback work fine?
It is possible that you could correct the Video playback issues while keeping the Video playback device set to "Same As Audio", by modifying this setting. If "vanilla" WASAPI doesn't work, then try WASAPI - Event Style (or vice versa). I don't know if the ASUS provided drivers are any good at ASIO, but they do support ASIO on those drivers, so you could maybe go that route as well. Also try with and without the "Play Silence for hardware sync" option enabled.
Lastly, make sure you have the current drivers for your card. I just looked, and the current driver for the Xonar cards was just released on 2010/10/29. So, if you haven't updated in the past two weeks, you're on an old driver. There is also a special beta ASIO driver on ASUS's website, which looks to be older, but is listed as: "Xonar Essence STX bit-perfect ASIO driver (beta) for Windows 7 32/64bit". There appears to be some special needs with this driver, though, so you may experience issues.
PS. I'm no ASIO/WASAPI expert, so someone else with more knowledge might want to chime in here... I'd also really like to get it working correctly on my system at home.