To be sure, what specific control panel settings are you referring to? If the problem suddenly occurred I would suspect a setting change in the Windows volume control sndvol32.exe. You could virus scan that file (in windows/system32), it can also be an autodialing trojan.
For both speakers to diminish the same volume, it has to be in the common audio path. Most faults results in no sound or just one channel.
Starman*