I know the only version Freddie recorded, but while I was listening to Mother Love, right at the end, Freddie starts singing Going Back but the voice sounds different compare to the original version. It sounds like his voice is from the 80's, maybe it's just me, but I though it can be possible he recorded twice as a rehersal, who knows...
While I have no opinion on whether or not this was rerecorded, I do know that Serpent is not the only one with this opinion. I have heard it a million times.
Again: there's one version of Goin' Back - I don't know about if it was remastered, remixed, played with wrong speed or something like that, but still there's one version of the song. Though Serry could be wrong, I think it's not that case.
It's most likely slowed down without doing much damage to the original voice. The effect is quite simple, It's not even really hard to do. if you have a good professional audio editor like Adobe Audition or something.