I'm sometimes of the opinion that the "it's a person's different voice on the soundtrack release" is some urban legend. It sounds exactly like the voice singing The Sword of Damocles and the mono version of Rose Tint My World. There is a lot of reverb on it, and yes, the verse is performed very differently. But the voice itself sounds the same to me. Susan Sarandon performer her verse differently in the same song. Patricia Quinn performs her verse of The Time Warp radically differently (much more sung on the sountrack, rather than the sing-speaking in the film). But I think with the Rocky voice as with Sarandon and Quinn, we are hearing the same performer in both cases.
There are weird false Rocky legends that pop up, which is why I don't quite buy this. Here are three more: 1) Shock Treatment never opened as a regular feature, it went directly to midnight. Wrong. It played here in Houston, Tx as a regular feature It played, for one, at the Town and Country 6. There were t.v. and print ads for it. If someone wants to check through old newspaper records, feel free. I know it was running, because I was planning to go to an afternoon screening, then I made the disastrous choice to see Saturday the 14th, which was playing at the same theater at the same time, instead (to be fair, this was two years before I saw Rocky itself, and I was only 12). Maybe the release was very limited, but it was released as a regular feature, first. 2)The prints with Superheroes that play at midnight always used the stereo remix. Wrong. When the VHS was released, and the prints in theaters were swapped out for ones featuring Superheroes, at least one of them was a mono print, because we got a mono print at the Bel-Air in Houston. I was told by someone who attended the big screening party at Fox right before the VHS release that the movie was screened there with a stereo print, but that the sound quality was so bad, the decision to send stereo prints out was junked. That mono print lasted for many years after that, even after the movie switched to the River Oaks. I was sick of the midnight phenomenon at that point, but would go occasionally just to hear the correct soundtrack again (and I'm obsessive enough about the mono track that I wouldn't have mistaken one for the other). Sometime in the mid-to-late 90's that print was swapped out for a stereo print, I couldn't say when exactly. 3) Henry Woolf, the shorter male Transylvanian with the fez isn't in the Time Warp. Theoretically, he didn't join the shooting until after that number was performed, and an obviously different, very tall actor stands in for him. Nope. Woolf can be seen very clearly--at times. He is, after all, shorter. He's best noticed when the Transylvanians are that one diagonal line across the ballroom. There just happens to be a tall Transylvanian with a fez among the cast as well.