Freedom of speech only refers to government regulation of speech. And, assuming that the speech in question here is protected under the 1st Amendment – and it may or may not be – there is nothing to stop a private organization from censoring to its heart’s content.
Freedom of speech does not imply the right to force one’s speech on anyone (this is the mistake the Dixie Chicks made; they ran up the flag of “freedom of speech” after their intemperate comments on the President while in the UK). It simply means that by speaking in an unpopular way, the authorities may not throw you in jail, or the like. In short, one may have the right to say something, but there is no right to a specific forum to say it.
If Memphis in May, or any other venue, wants to keep Three 6 Mafia off its stage or air, it's their call. But, if the FCC wants to get involved to censor lyrics thought merely “offensive,” that’s a different issue, one that we should oppose vigorously.
DE