I have just begun messing with Custom Music & Chants for FIFA and, like NCAA with Custom Stadium Sounds, I've found that it adds a great deal to the immersion and atmosphere.
I just wanted to share the site below because, like the site I use for NCAA sounds, this site is compatible with the PS3 web browser so you can download the sounds directly to your system without having to use a flash drive to transfer them from your computer.
You have to register with the site in order to download but that is as simple as can be (and free, of course).
The great thing about the feature in FIFA is that you can select a Playlist from your system to use for any of the Events. So for my Chants event for Chelsea, I have 15 different chants in a Playlist so the game cycles through them throughout the match and I hear a variety of chants. Very cool.
It's all very simple, but feel free to ask if you have any questions.