For something to be subjective, it must be open to individual interpretation. The presence of monotone is not. It has a very specific definition; a constant voice level that doesn't waver. When Natalie says "I will not give in to this", her voice level is constant. Ergo, if someone said something like "She sounded excited when she said that", they'd be absolutely wrong, because monotone by definition does not encompass changing voice levels, which are what we use to determine emotions expressed through vocals.