The answer is : "The Little Boy That Santa Claus Forgot," by Vera Lynn
Here is a site I found to give you a little more detail.
Quote:
I'd like to address the non-Floyd song vaguely heard in the background of the movie's opening section. It's called "The Little Boy That Santa Claus Forgot," by Vera Lynn (see "Vera" later in this analysis). The lyrics are as follows: "Christmas comes but once a year for every girl and boy/ The laughter and the joy/ They find in each new toy./ I tell you of the little boy who lives across the way/ This fella's Christmas is just another day..." At this point, the vacuum cleaner whirs into electric life and "When the Tigers Broke Free, Part 1" begins. After the song ends and we get that wonderful close-up of the Mickey Mouse watch, Vera's song continues with : "He's the little boy that Santa Claus forgot/ And goodness knows, he didn't want a lot./ He sent a note to Santa, what he wanted was a drum/ This broken little heart when he woke and he hadn't come/ In the streets, yes he..." Once again, the vacuum drowns out the song.