And they are singing what seems to be a new song before the throne and before the four living creatures and the elders, and no one was able to master that song except the 144,000, who have been bought from the earth.
NWT 2013 is apparently translating ὡς as "what seems to be."
"And they sung as it were a new song before the throne, and before the four beasts, and the elders: and no man could learn that song but the hundred and forty and four thousand, which were redeemed from the earth" (American KJV).
"and they sing as it were a new song before the throne, and before the four living creatures and the elders: and no man could learn the song save the hundred and forty and four thousand, even they that had been purchased out of the earth" (ASV)
Compare the Douay-Rheims and the ERV.
Henry Alford writes: "And they sing [as it were] a new song (i. e. if the ὡς be retained, they sing what sounded like a melody unheard before."
But some think ὡς should be omitted; NA28 places ὡς in brackets.