You can't, at least not without some kind of upmixing program. Sometimes this is included in the drivers, sometimes it's not, and if it's not, then you're just kind of SOL unless you have an audio file that has more than 2 channels worth of data in it.
My moms computer has surround sound. She is using Windows XP with AC97 6-channel audio hardware. I have installed the entire RealTek software driver and configuration utility package. The RealTek drivers are working and I am able to hear the self check tones through all available channels.
I then went through the windows volume setup and made sure that it also was able to identify and modify volume signatures for each available channel, and they are all identified and modifiable.
Now, how do I tell windows to allow me to use these channels? I would like the audio output to be piped to all available channels all of the time for all application.
Can Windows do this? Linux can.