I don't understand it at all but I occasionally get the same thing. The only thing I've found to work is to open Sound in Control Panel, make sure the speakers are set as the default and then select Configure for them. Then just start different variations of the settings. I have a 7.1 so I'll select that and check each speaker and, if they work, close out and Reboot. Sometimes I'll get sounds for some things and not for others but over time, everything mysteriously comes back. If that doesn't work, I'll go back in and change the settings back and forth (scientific isn't it?
). I also have a power switch for the speakers and sometimes just turning that off and on helps.
Just some ideas. It's all a mystery to me.
I have the same sound chip and software as you.