I've flip-flopped on the mundane/sorcerous singing. I don't believe the metagnosis can, by it's very nature, escape whatever trigger the agonic collar has, since to my understanding it's simply the addition of a second inutteral.
Only thing I can think is that Kellhus designed a spell specifically to break a collar, and taught it to Serwa as a contingency. An inutteral that masks the others maybe.