The question "what would you do if I sang out of tune?" is a line from the song "With a Little Help from My Friends" by Joe Cocker. The song goes on to say "Would you stand up and walk out on me? Lend me your ears, then Ill sing you a song. I will try not to sing out of key". The song is about friendship and the idea that friends will support each other even when things are not perfect. Therefore, the answer to the question in the song is that the friend would not walk out, but instead would lend their ears and support their friend.