via smyletube..."The Great Gig in the Sky" is the fifth track on The Dark Side of the Moon. The studio version features voice instrumental music by Clare Torry.
The song began life as a Richard Wright chord progression, known variously as "The Mortality Sequence" or "The Religion Song". During 1972 performances of The Dark Side of the Moon song cycle (prior to the album being recorded), it was simply an organ instrumental accompanied by spoken word samples from the Bible and snippets of speeches by Malcolm Muggeridge, a British writer known for his conservative religious views. By the time the band came to record the song for Dark Side, the lead instrument had been switched to a piano rather than an organ. Various sound effects were tried for the track, including recordings of NASA astronauts communicating on space missions, but none were satisfactory. Finally, a couple of weeks before the album was due to be finished, the band decided to try having a female singer "wail" over the music.

Spoken part:
"And I am not frightened of dying. Any time will do, I don't mind. Why should I be frightened of dying? There's no reason for it -- you've got to go sometime".
— Gerry O'Driscoll, Abbey Road Studios janitorial "browncoat"

Quotes from those involved:
Richard Wright:
Great Gig in the Sky? It was just me playing in the studio, playing some chords, and probably Dave or Roger saying "Hmm..that sounds nice. Maybe we could use that for this part of the album." And then, me going away and trying to develop it. So then I wrote the music for that, and then there was a middle bit, with Clare Torry singing, that fantastic voice. We wanted something for that bit, and she came in and sang on it.

Roger Waters:
It was something that Rick had already written. It's a great chord sequence. "The Great Gig in the Sky" and the piano part on "Us and Them," in my view, are the best things that Rick did -- they're both really beautiful. And Alan [Parsons] suggested Clare Torry. I've no idea whose idea it was to have someone wailing on it. Clare came into the studio one day, and we said, "There's no lyrics. It's about dying -- have a bit of a sing on that, girl." I think she only did one take. And we all said, "Wow, that's that done. Here's your sixty quid."

David Gilmour:
Clare Torry didn't really look the part. She was Alan Parsons' idea. We wanted to put a girl on there, screaming orgasmically. Alan had worked with her previously, so we gave her try. And she was fantastic. We had to encourage her a little bit. We gave her some dynamic hints: "Maybe you'd like to do this piece quietly, and this piece louder." She did maybe half a dozen takes, and then afterwards we compiled the final performance out of all the bits. It wasn't done in one single take.

Clare Torry
I went in, put the headphones on, and started going 'Ooh-aah, baby, baby -- yeah, yeah, yeah.' They said, 'No, no---we don't want that. If we wanted that we'd have got Doris Troy.' They said, 'Try some longer notes', so I started doing that a bit. And all this time, I was getting more familiar with the backing track. ... "That was when I thought, 'Maybe I should just pretend I'm an instrument.' So I said, 'Start the track again.' One of my most enduring memories is that there was a lovely can [i.e headphone] balance. Alan Parsons got a lovely sound on my voice: echoey, but not too echoey. When I closed my eyes---which I always did—it was just all-enveloping; a lovely vocal sound, which for a singer, is always inspirational.

On the DVD Classic Albums: Pink Floyd -- The Making of The Dark Side of the Moon, various members mention that they had this song and weren't quite sure what to do with it. Wright further mentions that when she finished, she was apologetic about her performance even though those present were amazed at her improvisation