Some "lost" (now found) tapes reveal Apollo Astronauts heard some pretty unexplainable 'music' on the far side of the moon. link
Sadly, they were on the far side of the moon and not the Dark Side of the Moon, so it wasn't that sweet of a jam.
In all seriousness, the crew were understandably startled when they encountered strange, music-like radio transmissions though their headsets. At the time of the occurrence, they were unsure whether or not to report this activity to NASA.
Here's a link to what the tapes reveal, in case you're interested.
According to the forthcoming Science Channel series (NASA's Unexplained Files) the recordings contain "strange, otherworldly music coming through the Apollo module's radio".
It reportedly lasted about an hour.
Here are some recorded quotes from the astronauts: "It sounds like, you know, outer space-type music", "You hear that? The whistling sound? Whooooo!", and "Well, that sure is weird music!".
It seems all the listeners unanimously agreed that it was music. However, because this happened decades ago it would be difficult to find out exactly why they thought it sounded like music.
They regained contact with NASA and went back and forth on if they should tell NASA. But this begs the question...why wouldn't they tell NASA? Weren't they used to reporting every minuscule detail of their lives in space? Why would they leave out this strangeness?