You can try Spirit FM Radio Unlocked from the market. I tried it, but it didn't seem to work for me. Can't recall if I was using ctmod at that time or not.
You can always try it out and get a refund if it doesn't work. The developer claims to honor refund past the 15min market return period.
More than a claim, it's a promise.

I cancel any order at anytime (even a year later) on request. And 99% of those requests are within a week and just because of device/app incompatibility. I recognize that almost anyone can pirate if they desire, and my value is in the support, which is often tricky on custom ROMs (my target market).
There's also a free version linked here, and lots of info:
http://forum.xda-developers.com/showthread.php?p=13379669
BTW, I stumbled on to this thread; was looking for LS855 rooting info for a potential customer, saw an "FM Radio" thread and couldn't resist.
BIG warning though, I'm no longer officially supporting any LG devices, except the P500, mostly because I own one. I've had almost no luck figuring out ICS/JB ROM FM audio remotely (without a device) for almost every LG device I've supported in the past.
Spirit FM Radio (version 1) is now considered "mature" and I'm currently designing Spirit2. More info on the XDA thread I linked, and please ask me any further questions there; I can't promise to come back here.
Thanks !
Mike.