Neil Armstrong was confronted with the recording, having always maintained he said “a man”, and he admitted in TV he did not hear it either. He still insisted being sure having pronounced it.
Decades later, someone deconvolved the soundtrack (remember there is about 2 seconds delay and sound transmission was nothing like hifi) and claimed the “a” was there. I am not sure if anyone did a serious analysis of the voice processing, which would likely be inconclusive.