Sometimes it is indeed simple absent-mindedness, being caught up in the music, or flight anxiety. Politely inviting the vocalist's attention to the singing may be all it takes for him or her to stop.

Airplane karaoke?

Or if that doesn't work, and it is a song I know, I may accompany in a loud Naval Academy Catholic Choir baritone voice. 🎶 This could be enough to gain a friendlier seatmate and get the flight attendant call buttons ringing.