Another point ... "Happy Birthday" is disputed as being the first song sung in Space, although it is often quoted as being so.
According to the transcript of his flight, Yuri Gagarin sung "The Motherland Hears, the Motherland Knows" by Shostakovichon during his re-entry on 12 April 1961. Of course, Gagarin's flight predates Apollo 9's flight by a few years.
The 'Guinness Book of Records' is often quoted as the source of the "incorrect" information but they have stated to clear up the matter (on 25 August 2006) "We have made reference to Happy Birthday being a song sung in space by members of the Apollo IX mission, but it is not referred to as the first song sung in space, it listed as anecdotal information to a record about the most frequently sung songs in English."
Other interesting "music from space" ~
As well documented in the film 'The Right Stuff' and various documentaries, John Glenn was a very religious man and hummed hymns when under stress (but resisted actually bursting into song).
Jingle Bells was also played (but not sung) by the Gemini 6 astronauts as a prank ...
"On December 16, 1965 performance art made a great leap forward. US Astronauts Walter M. "Wally" Schirra Jr. and Thomas P. Stafford were preparing reentry into the Earth's atmosphere after successfully piloting their Gemini VI spacecraft within 30 centimeters of Gemini VII, still in orbit. As they maneuvered to begin reentry, Stafford contacted Mission Control about a UFO:
"We have an object, looks like a satellite going from north to south, probably in polar orbit.... Looks like he might be going to re-enter soon.... You just might let me pick up that thing.... I see a command module and eight smaller modules in front. The pilot of the command module is wearing a red suit."
Before Mission Control could respond, Schirra began playing "Jingle Bells" on a harmonica he had smuggled in his spacesuit. He was accompanied by Stafford, playing smuggled sleigh bells.
The first musical instruments performed in space are on display at the National Air and Space Museum of the Smithsonian Institute in Washington, D.C.
Source - Article from Smithsonian Magazine