Some 10-15 when I was a kid I used to watch a cartoon movie about a kid who was in an underground world of monsters. I can't really remember how he got there, but I do remember that these monsters were afraid of singing. When the kid sang, the monsters couldn't stand it.

I've searched a lot on the internet but I just can't find the name of the movie. It's not "Little Monsters", the movie I'm searching for is a cartoon.

Sounds like The Princess and the Goblin (http://www.imdb.com/title/tt0107875/)

Yes, that's it! Finally :) Thank you sooo much!