It was not true. The incidence was fabricated and a hoax. This is why Neil Armstrong denied it. . There is no evidence whatsoever to support the claim that Neil Armstrong converted to Islam, or that he and the other Apollo 11 crew members witnessed anything miraculous while on the moon. Conversely, there is copious amounts of evidence that directly or indirectly prove the entire claim to be a hoax. The evidence against this claim is so strong that there has even been a fatwa issued confirming it as fabricated. Armstrong himself, in his autobiography, denied ever converting to Islam. His administrative aide and the US State Department also denied any truth behind the conversion rumors. He would later, in Malaysia, explicitly deny that there was any truth behind the claim that he also heard the Muslim call to prayer on the moon, and the transcript and audio of the moon landing itself contradict the claim that ""strange" sounds or words were ever heard. http://www.wikiislam.net/wiki/Neil_Armstrong_(Conversion_to_Islam)
if he said he didn't hear it then he probably didn't since only himself and buzz aldrin were the only ones up there. someone else saying he heard it is considered hearsay or out of law term a rumor. Unless it is bound in writing or recorded of his acknowledgment he heard it.