Hello, Haruka. Your first sentence is perfectly fine (I would change "which" to "that" as it sounds a little more natural). If we want to stick to the structure you provided (with "of" being removable), other ways of expressing the same idea would be: "Let it Go" is sung in the movie "Frozen." "Let it Go" is sung by Idina Menzel in the movie "Frozen." "Let it Go" is a song from the movie "Frozen." "Let it Go" is from the movie "Frozen." (This one will require some context to make sure the reader already understands that "Let it Go" is the name of a song.) I hope this helps =)

You can use the word 'soundtrack' to refer to the music and songs that are used in movies and television programmes. For example: "Let it go" is a soundtrack from the movie Frozen. "Let it go" is a Frozen soundtrack.

Coeus's answer is not correct in the US. All of the songs from a movie are called the movie's soundtrack. So you could say "'Let It Go' is on the soundtrack from the movie 'Frozen.'" Or you could say "That movie had a great soundtrack" if you like all of the songs that are played in the movie. Going further, songs that are written specifically for a movie musical or a musical play are called "showtunes." (This is more true for a play, but it can be used for movie musicals like "Frozen" as well.) So you could also say "'Let It Go' is my favorite showtune." If a song is not written for a movie, just played in the background, it is not a showtune.