Go to control panel, sounds and audio, in speaker settimgs click on advanced and select the type of speaker set up that you have ie 5 to 1.The click on sounds tab and select windows default. Next click on Audio and select the correct sound card from the drop down list. Then select voice and check that your sound card is visible in the drop down list. Click apply. In the Device area, click on advanced and check the setting in the control panel, if all appear OK, make sure that the mute all is not ticked. Click OK and try it. My system seems to reset these on a regular basis.
Let us know how you get on.