Well I've run this through every concievable filter, time stretching algorithm and noise reduction scheme I could think of and it's hard to definitively say that the "a" is in there. I time stretched a clip of the words "for a man" slowing it down. Here is a screen cap with the area where the "a" could go highlighted:http://i113.photobucket.com/albums/n211/apollolanding/Spaceflight/smallstep.jpg" width="720" border="0" />
Listening only to "for a man", slowed down, it's easier to hear or imagine to hear the missing "a". When you listen to the entire statement, the cadence doesn't lend to the "a" being there. There is static that could be attributed to com loss but it seems too quick to be a syllable and is a separate event not slurred to the word "for" like "fora man". Here's an .mp3 of exactly what you see in the screen cap above. I just don't hear the "a". I believe he intended to say it but even the first man on the moon is allowed a little slip up. Heck I always thought "Houston, Tranquility Base here. The Eagle has landed" was a much cooler statement...
g4ayu, that was an edited clip on Eric Jones' Apollo Lunar Surface Journal website. It was created to show what the sentence would have sounded like had Neil said the "a". Here is the explanation from Mr. Jones:
109:24:48 Armstrong: That's one small step for (a) man; one giant leap for mankind. (Long Pause)
[As Andrew Chaikin details in A Man on the Moon, after the flight Neil said that he had intended to say "one small step for a man". Andy and I agree that the flow of the dialog at this point in the tape suggests that Neil forgot to say the "a" and that there is little likelihood that the "a" was lost in transmission.]