Who sings this song vs who sang this song
We all know Michael Jackson is not alive.He died on June 25,2009.Suppose,I am listening to a song like "Beat it (by Michael Jackson)" with my friends and I don't know the singer.After the completion of the song, if I question my friends then what will be the questions:

1.Who sings this song? 2.who has sung this song?
3.who sang this song?
4.who is singing this song?

I am confused.Please get me clear.

Question

Who sings this song vs who sang this song
We all know Michael Jackson is not alive.He died on June 25,2009.Suppose,I am listening to a song like "Beat it (by Michael Jackson)" with my friends and I don't know the singer.After the completion of the song, if I question my friends then what will be the questions:

1.Who sings this song?                                                                                                                       2.who has sung this song? 
3.who sang this song? 
4.who is singing this song?

I am confused.Please get me clear.

Answer

I'd say who is singing, because you're listening at this moment. And if you ask this question it means that you don't know that its Michael Jackson's song, so the singer might be still alive. So your question is " who is singing in this recording ?"

Answer

In my opinion:
“Who sang this/that song?” or “Who is/was the singer?”/“Who is/was the singer of this/that song?”
The reason why both “is” and “was” in “Who is/was the singer?”“ are ok, is because the singer is still the singer after the song stopped playing, but he also was the singer when the song played.
You wrote it was after the completion of the song, which is the reason why I 
would say "sang" and not "singing". Had it been while the song was playing I would have used "is singing".