If you're referring to the version recorded July 25th, 1973, for Queen's second BBC session, then yes, I've heard it too. It's a quiet voice which comes in as the opening percussion starts. I'm not sure if is appears on every copy, but it appears on mine.
I'm hesitant to say it's Brian or anyone else related to Queen, since the source of this recording is unknown. It could be any of the following:
a) a stray bit of DJ vocal from the radio broadcast
b) some radio interference picked up by the machine making the recording (we get stray radio signals from time to time in the theatre if there's an unshielded ground wire somewhere in the set-up running into the console)
c) a left over bit of unrelated vocal from one of the generaions of copies made before it reached us, which somehow got mixed over the beginning.
d) an engineer in the studio present at the original recording
e) a band member muttering
Really, it's hard to tell what he's saying, or if it's even English, so unless it appears on the original master or a cleaner copy surfaces, we may never know.