Sounds like your drivers were recently updated/mucked with. Could be something as simple as a Windows update screwing with them.

Find your specific sound card model, and look up the settings for how audio switches from device to device.

Another test: Don't just do movies. Try music in Winamp or windows media player, iTunes, etc.

Make sure that you have multiple programs to test with, not just WMP, or VLC, etc. Then you know if it is a program specific issue, or a general drivers setting issue.