Just asking but how was their "free speech" violated? They chanted right? Nobody stopped them. Nobody said they couldn't sing the chant. So they chanted. There was no violation of their right to free speech.
There is some aftermath of them exercising their free speech. But that's not a violation, or denial of a right, that a repercussion of free speech.
If you exercise your right to free speech, which they did, then you better be ready for a reaction....which they weren't.
A university that takes government money becomes a state actor under established First Amendment doctrine, and universities are a classic public forum. By expelling them, Boren punished them for their viewpoint, which is impermissible. Further, the speed with which everything has gone down raises serious due process questions; I highly doubt that the fraternity, or any of its members, had the opportunity to have a hearing before a neutral arbitor.
I don't know that anybody will actually bring suit--they're definitely not sympathetic plaintiffs--but if they could get a fair hearing, I'd lay odds on them.