I never assumed you wouldn't notice, but I've just learned through trial and error that no freeware ac3 encoder can output a high quality 5.1 track, but they can encode a very high quality stereo track because the bit rate can go as high as 384kbps for 2.0 channel and even free encoders handle that nicely.

Vegas is such a bitch to work with that I only use it for the final step - the encoding.

I listen pretty much only to film music. When I try to upmix a soundtrack album into 5.1, I try two approaches depending on what is most appropriate taking the album's stereo mixing into conderation:

-attempt to recreate the discrete 5.1 mixing as best I can by hearing it within the film. Often with scores recorded in England (Abbey Road/Air Lyndhurst recordings) have a concert hall reverb to the rears, and aren't as discrete as Hollywood recordings tend to be.

-create a slightly more discrete Dolby ProLogic II upmix effect, adding a slight delay to the rears.

I find if a score has a lot of ethnic percussion and woodwinds, that makes for a more discrete mix than the average orchestral score since those instruments tend to be limited to the center channel and therefore don't bleed through to the other channels as much.