Ok guys was messing about with the custom chants on FIFA 11 today and it is superb, heres how i done it.

Firstly I went onto youtube and found videos with the chants that I wanted, I then downloaded this: http://www.dvdvideosoft.com/

and used the Free YouTube to MP3 Converter
to download my selected videos and convert to MP3.

I then opened the MP3 with Audacity audio editor http://audacity.sourceforge.net/
and edited my chants accordingly fading them in and out and upping the volume on some. I then exported them individually to WAV files and burnt to disc as an Audio CD

Pop the Audio CD in your 360 and play the disc, then select rip CD and give each track a name.

Next play the first ripped track and then create a playlist adding the other required tracks.

Save the playlist and start FIFA 11, go to my FIFA 11 and select Custom Chants, and select fav team, then simply load your playlist for whichever chant you want to customise, I have the crowd cheer customised with my playlist of spurs chants and it works really well....enjoy!