You should have control software on your PC. It will be located under your start menu. You may have to search for it. If you cannot find it, it may have been removed accidentally at some point. You should be able to find the latest drivers on your PC's manufactures web site or directly on Realtek's web site. http://www.realtek.com.tw/
Once you open the software there is a default setting button along with other adjustments that can be made to the sound.