Right click the speaker in the lower right of the screen, select Sound.
On the Sound pop-up, select Playback tab.
Select Speakers, then click on Properties.
On the Speaker Properties pop-up, click the Levels tab.
The scroll bar should be on the right for all speakers and input the system recognizes.
Line-in, Microphone, Side, Front, Rear, Center and Subwoofer or whatever you have. Each of those have a slider to adjust level.
But...I don't know how you're getting 5.1 if you only have two speakers.
And, I'm assuming you're using the Realtek drivers.