Menzel performed the hit song "Let It Go" in the original Frozen (2013), and takes on the new song "Into the Unknown" in the sequel.
"There are seven new songs, amazing songs," Menzel said. "This one has the most heartbreaking song."
Bell and Menzel responded to people calling "Into the Unknown" the new "Let It Go," saying there is another, unheard song that may rival "Let It Go."
"Look, here's what happened," Bell said. "She did it the first time, and Disney was like, 'Should we double down on this?' Everyone was like, 'Yes!' So they gave her two ... There are two unbelievable [songs]."
"I sang them on a good day, which means they have really high notes," Menzel added. "It's going to be so hard on the day when I have a cold and I have to go sing it somewhere."
Panic! at the Disco released its version of "Into the Unknown" for the Frozen 2 soundtrack this month. Menzel's version will appear in the film, which opens in theaters Nov. 22.
Bell previously said on Jimmy Kimmel Live! that Frozen 2 has "matured with its audience." Josh Gad, who voices Olaf in Frozen and Frozen 2, said in October that he "wept" while seeing the sequel, calling it a "stunning and surprising" film.