Thanks for the responses so far, folks. They are in line with what I've surmised from my long experience, but I haven't done any actual research.
Of the "sweetening" sounds added post-'77, the surveillance camera explosions and the asteroid swooshes really stand out, bug me no end, and pull me right out of the 1977 euphoria granted me by the opening crawl - which, if it is a fake, is a darn good one. My gut tells me it's a fake, btw.
Of course, my gut also told me that 3PO's tractorbeam rant and a stormtrooper's "Close the blast doors" were in the 70mm, stereo mix that I saw many times June thru September in '77. After that, thru the end of the year, I saw the film in a second-run house that must have had the mono mix, and that's where I must have picked up those lines of dialogue. Hence, I will forgive their absense from the newly released DVD.
I wish those later-added sound effects didn't bug me so much. Other folks are bugged by the ghosting, the grain, the non-anamorphicality ... but I am only bothered by the non-seventysevenness in a product labled "original theatrical release." Bah.
(oh well, to each his own quibble.)