Just like Sonny V. said, tell the dinning room staff. I have seen them singing Happy Birthday and Happy Anniversary to passengers at other tables. All of the dinning room staff all come from a mix of many different countries: India & Central Asia, The Far East, and Eastern Europe. I am not sure if they sing off-key, but it sure is interesting to hear all those foreign accents singing:
"Hepy Burt Day To Jiu, Hepy Burt Day To Jiu"
I suppose, in all fairness, it might be funny for them to hear us singing a song in their native language.