In the Nether Fortress level there are two doors that lead you to get the Key Golem to continue with the level, but when you go through the doors it should change the background music to the song "Humus-Sifting", listen to it here to check it: 

https://www.youtube.com/watch?v=gcy1UkAtDAA&list=OLAK5uy_k2PTSYQvBNXZ9I1Nury7xA4z23p6YWT2c&index=19