Winamp has several plugins available that will allow you to do that. Use the Linein plugin and then something like Allesandro Tomassini's Orban Optimod processor plugin and you have a nice FM processor that behaves very much like a real Optimod. Not sure about AM, but you could do a search and see if there's something that would work for you. I haven't found anything of the sort for Linux, though Winamp does run on Linux using Wine.