His voice.
His voice, that deep baritone that would send chills down his spine, much as the cello does when scaling to its climax note, was what John missed most about Sherlock's absence. If it had to be narrowed down to one thing, it was that.
It's been one of those days; a day where John spent the entire day working, every spare thought that wasn't focused on saving a patient was filled with Sherlock, Sherlock, Sherlock. A day where John walked home, musing about how he felt like he had more purpose in helping the infuriating consulting detective—the only of its kind!—than saving lives in the hospital. A day where he sat in front of the telly, wishing that Sherlock was there to complain about yet another episode of Doctor Who or whatever was on that John just so happened to like.
And yet, after he shut the telly off, after he prepared himself for sleep and walked past the room that had once been Sherlock's, he found his mind asking, What do you miss most about him?
Oddly, the answer had been a quick one, an immediate response.
His voice.
To some people—if he ever chose to admit that, let alone bring Sherlock up in a conversation—that might seem odd. Odd, that he missed Sherlock's voice more than his presence, than his constant cases and constant complaints. Wouldn't you rather to not hear his voice anymore? they might ask. Wouldn't you rather he just be there, not nagging or complaining or insulting? That would certainly be a logical question.
And yet, nobody realized how much he missed the nagging and complaining and insulting. While, at the time, it had been an annoyance, it had been Sherlock's voice. And—hidden in between the rude remarks and exasperated exclamations—Sherlock presented compliments. There were always little comments, little remarks about how much more potential John had than anyone else.
Sherlock was still self-centered, but he cared more for John than anyone else. Any passerby could see that if they only observed as Sherlock did.
And, even more rare—but what a blessing they were when they surfaced—Sherlock would let escape a tiny moment of humanity, give a hint at the quickly blossoming emotions that had been planted upon John's arrival. It might be nestled in an offer for tea. It might lurk in his occasional consolation when John was struggling to recover from an extremely trying case. It might even be publically presented in a proclamation of John's brilliance.
It didn't matter how sparing these little moments were; they were Sherlock's little way of showing how much he cared for—and possibly loved—his flat mate, all given vocally.
So yes, John missed his voice. John missed how much he had fallen in love with that tone the first time Sherlock had spoken, the magic that was spun as he deduced John's past. He missed the connections he drew to their relationship when he heard it, the frustration and happiness and exhaustion and love all tied together in one single sound.
It was beautiful, painfully beautiful, the kind of beautiful that reminded one of the torture and happiness that love could bring about.
That beautiful thing was gone from him now, taken away by the man who possessed it. The thought of never hearing it again—even though he's been aware for three years of its absence—is too overwhelming for John to bear.
He's crying now, realizing how lost he is without Sherlock's voice to guide him. There's no way he will sleep now, and he grudgingly turns around to revisit the telly, to watch another episode of whatever was on, to drown in how mundane his life had become without Sherlock.
It's that thought that nearly breaks him, but the sound of knocking at the front door stops the damage.
And, on the other side of that door—the door that leads to 221B on Baker Street—is the voice, ready to break the silence.