According to declassified transcripts from the Apollo 10 mission (May 1969) the mission crew, picked-up a strange music on their receivers while orbiting over the dark side of the Moon. This music was not described as Earth-like and could not have been transmitted from the Earth as while orbiting over the hidden side of the Moon communications with Earth were lost. The mission was manned by three experienced astronauts Thomas Stafford; John Young and Eugene Cernan. These scientists were knowledgeable enough to distinguish between a radio interference and music.
It all happened when the lunar module was orbiting the the Moon. While on the dark side of the Moon, it meant that telecommunication signals could not reach the crew since the Moon stood between the lunar module and Earth. They had no communication with mission control in Houston for approximately two hours. During that time of silence they started to pick-up strange music.
There is a high degree of certainty that this event took place since although NASA has never officially published the actual audio recordings it has disclosed a transcript from the recordings of the spacecraft’s black box documenting the discussions of the astronauts about the music they picked up. The extracts that follow are from the official NASA transcript.
Abbreviations CDR, CMP and LMP refer to Stafford, Young and Cernan respectively while LM and CSM refer to the lunar module and the secondary module which was separated to conduct a lunar landing rehearsal. At the time the discussions over the strange music took place, Young was in the secondary module (CSM) which was at a distance from the lunar module.
The first evidence indicates that the experienced astronauts identified the sound as music. The discussion includes all three astronauts.
The transcript indicates that the astronauts are picking up the music from both spacecraft. Also in line (04 06 13 34) Young is asking Cernan to look at something which is outside that could be the source of the music. If the testimonies of astronauts seeing unidentified craft in space are true then we may be looking at another case in this transcript.
A few minutes later the crew keep talking about the mysterious music.
On their next orbit over the dark side the music returns
The music comes back again and again. There is no doubt on Cernan’s mind that its music and not some random radio interference.
Finally, the astronauts comment that the music suddenly stopped.