"The 144,000 people sang a new song." This explains what the sound was that John heard. Alternate translation: "That sound was a new song that they sang"
# No one could learn the song except for the 144,000
This double negative emphasizes that the 144,000 were unique. Alternate translation: "The only ones who could learn the song were the 144,000" (See: [[rc://en/ta/man/translate/figs-doublenegatives]])