If one of your default sound devices in Windows is something other than the device MC uses, you will get 2 volume sliders...
I have an audio zone and a video zone setup in MC. When I play video, the MC uses my onboard sound device (which is connected to my TV). When I play music, it automatically switched to my Xonar, which is fed to my amps. In Windows sound properties, my device is the onboard audio (so if I'm streaming video thru a browser, it's not pumped thru my amps...important because of annoying, sound playing popups!!)
Go thru your Xonar sound properties in Windows...by default, the Xonar drivers set volume of the card to 80% if max. You want it at 100% because you're using internal volume control. (Make sure you check option to maximize volume on sound device in MC options). Also, open the Xonar control panel and make sure you go thru all the setup pages to ensure vol is max and no sound effects are enabled.
Personally, if you're not using an audio patcher/studio with your soundcard, I'd forego ASIO in favour of WASAPI.
Hope this helps!