I'm no copyright/patent/trademark attorney-type, but if the contents of that letter can be believed and it's legitimate, I have to question Limbaugh's legal people in the use of that music for ANY reason.
Jeezus, guys, you can't use music like that unless you're LICENSED to do that. And that means, of course, you're paying a royalty for the privilege of playing that music.
Had they been licensed, Rush's attorney's might have recourse to pulling the license, but Limbaugh would be under no threat of a lawsuit for using copyrighted material illegally.
This is simply stupid.