To mutter under your breath means to mutter in a very quiet voice, but in this context the person has been shouting in a very loud voice
Consider the staetement "I'll testify how you've been going mad for weeks, shouting in a very loud voice and all that.". Based on this statement, is "I 'll testify how you've been going mad for weeks, muttering under your breath and all that." a true statement? Yes/no/unknown