The sound even startles him a little – which is ridiculous, as it was made by his own vibrating vocal chords – and he can almost see it in physical form as he says it. It's sort of like watching ripples of air rising from a heater.
He tries it again.
"Bored. Booooored. Boooooooooreeeeeeeeeeee….d."
He plays with the word – elongates the vowels and then the plosives until the word becomes more melodic than functional – and this amuses him for a short while, but there's only so much you can do before you start saying other words entirely (although he notes that it is only a small shift in the airflow through his nose that makes the difference between "mort" and "bored" and he finds this oddly appropriate), and he soon gives up his new toy and rolls over, pressing his face into the sofa cushions with the petulant wish to asphyxiate.
He sleeps instead, and when he wakes she is home, and she is watching him. It thrills him slightly, for he has always relished attention even without craving it, and he wonders for a moment if he should feign sleep to make her stay.
Irene, however, is once again cleverer than he gives her credit for, and he hears the rustle of her dress and feels the sofa sigh toward her as she leans down over him.
His ear feels her, alerting him to her proximity just before her breathing touches his skin, and in his mind's eye he sees the sound waves extend from her mouth to his ear just like he had seen them project "bored" around the room earlier and he wonders if his boredom has contributed to his new echolocation abilities and then he sees dolphins and fish and chips in New Quay – Wales, not Cornwall, but essentially the same – and bloody seagulls and the wind and the corpse that had all her fingernails missing and-
"You're adorable when you sleep," Irene murmurs then, almost smooth enough to be a sound wave singular rather than sound waves plural, and Sherlock can hear the teasing over the burst in his own heart rate, "it's rather a shame that you never do it. Perhaps I should drug you to keep you this way."
He turns his head so that he can fix at least one eye on her, her ever-red lips stark against her pale skin and the paler room, and he can see nothing of the day on her. She has showered since coming in, then.
Sherlock scowls when she does not say anything, for this means she has no new leads and he is so bored and frustrated and there are only so many times he can threadcount Irene's bedsheets under a microscope as if he hasn't already done it at least three times this week.
"Bored." he announces, though he's certain she already knows that if the look she gives him is any indication, and then she is suddenly gone. Sherlock twists and rolls awkwardly, enough to see her figure disappearing down the hallway, before falling off of the edge of the sofa.
She does not return for a few days, and he supposes she has better things to do with her time than spending it with a dead man.