This may sound simplistic, and perhaps you already check this (I can't tell by reading the post), but how are your speakers set up in the Control Panel?

=Go to Control Panel, then
=Sounds, Speech, and Audio Devices
=Click on Sounds and Audio Devices
=Near the bottom right next to Speaker =Settings, click on Advanced
=Under the Speakers tab, select the proper speaker setup if not already correct
=Click Apply