Yes Audrey Hepburn did sing happy birthday to President Kennedy, the year after Marilyn Monroe had.
Cite: Everyone remembers when Marilyn Monroe serenaded President John F. Kennedy on his birthday in 1962. What is often forgotten is that Audrey Hepburn sang "Happy Birthday Mr. President" to JFK for his final birthday in 1963.
"Hepburn Biography." 8/14/2009. <http://www.imdb.com/name/nm0000030/bio>.