That happens to me too. Sometimes the song has some sort of ultra-significant meaning,other times it just happens to be there. Familiar songs, too, but sometimes completely unknown or perhaps original ones...I do write songs a bit but it is not a regular pasttime of mine...the function of the songs sometimes seems to be to mark a certain moment or occurrence in the dream, and sometimes its perceived quality or specialness is forgotten or fades to mundane   after considering it in wakefulness...Just as sometimes a language unknown to me is used perfectly coherently in a dream...