Is there something simple you can think of that I may have overlooked?
Yeah!
From all you have written, the problem does not lies with Vegas, its with your system audio settings.
Try putting everything back to its default settings and start from there.
Steve Rhoden (Cow Leader)
Film Maker & VFX Artist.
Owner of Filmex Creative Media.
Samples of my Work and Company can be seen here:
https://www.facebook.com/FilmexCreativeMedia