So in scenario 2,
We know that Peter was killed being punched brutally by 1 of the 5 friends of Peter's cousin.
1) We know that Peter's cousin didn't kill Peter because he was the guy who found Peter's dead body.
2) We know that the 3 friends in the garden didn't kill Peter because they were at the garden the whole time.
3) SO WE KNOW that the 2 people(Sam, Jeff) left were those who were playing piano.
What happened.
Sam and Jeff have so much hatred towards Peter. So in the music room, Sam played the song loudly as to distract the mind of other people in the house making them think Jeff was also playing along the piano with Sam.
While Jeff was playing the piano, Sam brought Peter to master bedroom and punched him to death. Before Peter's cousin found Peter's body, Sam quickly left the room and brisk through the music room, closing the door.
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The End.