Okay, so after trying a few more things I ended up downloading hexxit 1.0.5 so that de Better Dungeons mod version was 1.0.8 (or 1.8, can't remember exactly). And now I have nether cities On the other hand there are a few sounds I'm not getting, like when you mine a block, or when you pick up an item; and hexxit crashed a few times. But it's alright, I will do some research and see if I can fix it. Thanks to everyone who read the post even though there weren't replies.
To everyone who experienced the same problem and don't know how to download an older version of hexxit, all you have to do is go to the Technich Launcher and click de little cogwheel on the down left corner of the Hexxit logo. There you must click the thrid option, which allows you to download the version you like. I tried 1.0.6 but the problem wasn't fixed, so I ended up trying 1.0.5 and it worked :)