NASA Declassifies Apollo 10 Transcripts - Astronauts Heard "Outer Space" Music

Making headlines lately have been reports
that NASA recently declassified documents 
and audio tapes revealing that Apollo 10 
astronauts heard music as the spacecraft
flew around the far side of the moon.

NASA has issued a response to the reports stating:

“While listed as ‘confidential’ in 1969 at the height of the Space Race, 
Apollo 10 mission transcripts and audio have been publicly available since 1973. 
Since the Internet did not exist in the Apollo era, we have only recently
provided digital files for some of those earlier missions.
The Apollo 10 audio clips were uploaded in 2012, but the mission’s
audio recordings have been available at the National Archives since the early 1970s”

As for the likely source of the sounds, 
Apollo 10 Lunar Module Pilot Gene Cernan said on Monday,
‘I don’t remember that incident exciting me enough to take it seriously.
It was probably just radio interference.
Had we thought it was something other than that
we would have briefed everyone after the flight. 
We never gave it another thought.’

A look at the transcripts…

We can conclude from the 
transcripts and the audio
that whatever it was they 
were hearing was different 
from anything they had 
previously experienced.

These were men were well
trained and experienced.
Apollo 10 was the first of
only two Apollo missions
with an entirely flight-
experienced crew.

What did they hear out there, and where did it come from?