Custom stages were introduced not long after the initial release of AAC.
While the original stage builder feature was basic and highly limitted, the latest version of AAC is so flexible that some users have attempted to make full campaigns using the builder, which was not its original purpose.
There are many AAC custom stages floating around the net, all you have to do is find them!
If you want to create your own stage, take a read through the included mannual.
Creating your own diverse stage is easy, just have some of your favorite sounds near by and begin to create!