The FM radio in my S1 did not work under custom roms either, I think the radio driver was propreitary and so seperate binaries were never available. I never tried but I was told an app called spirit FM could get it working again, check it out might work on your S2
Ok, downloaded a free version from XDA and it seems to work on my GS2 with CM10! Got triple J and a few other canberra stations. Its a paid app on the app store though too.
Lots of happy GS2 customers too in the reviews.
https://play.google.com/store/apps/...GwsMSwyLDEsImNvbS5taWtlcnNtaWNyb3MuZm1fbGkiXQ..
http://forum.xda-developers.com/showthread.php?p=13385163