Yamamoto starts laughing as soon as Gokudera begins to sing.
Gokudera told him already, has been telling him the entire way out to this ill-advised karaoke date. "I cannot sing," back when they left his house, "You don't want me to sing," as they crossed the street, "I'm not fucking doing this," as Yamamoto pushed the door shut behind them to leave them alone in the space.
"No way," he grumbles as he throws himself into one of the chairs, apparently there so one can be comfortable while mocking his friends. "Can't we just make out instead?"
"It'll be fun!" Yamamoto chirps without looking up from the screen he's messing with. Gokudera flinches at the delight under the words, the happiness audible in Yamamoto's voice because he knows he's done for against that tone, he was lost as soon as he huffed and agreed to this ill-fated expedition. "It's just me anyway." He's scrolling through the song options, pausing every so often like he's considering before continuing on. "It's not like you have to be embarrassed with me."
"I'm not singing," Gokudera says again, one last desperate try to escape the fate coming for him.
Yamamoto hums high in his throat, the sound of finding something he didn't know he was looking for. "Here!" He's pushing the button to select a song before Gokudera can even see what the title is, unclipping a microphone and passing it back in Gokudera's direction with no consideration at all for the other's denial. "You're doing the first verse."
"What?" Gokudera takes the offered microphone automatically, too distracted by the way Yamamoto is looking up at him to realize the implicit agreement in this until it's too late. "Wait, no, I'm not singing!"
"It's only a few lines," Yamamoto says as the opening bars start to play. "Then I'll come in with you."
"No," Gokudera protests, but the words are starting to scroll over the screen, insistent electronic pressure to counterpoint the delighted anticipation in Yamamoto's eyes, and the snap on the words takes the last of his resistance. He doesn't have time to formulate more of an argument before he has to turn to the screen, take a rushed inhale for the first verse, and attempt to form his throat around the resonance required.
"Hate to lose and always stubborn -" and Yamamoto is grinning, he's clapping a hand over his mouth to muffle a giggle and oh Gokudera is going to kill him in a moment. If it weren't for the lingering habits of a musician he'd throw the microphone at the other boy's head, but the words are clear and insistent and now that he's started he can't give up halfway, even if he can hear every flat note cringingly clear in his ears. He frowns instead, glares at Yamamoto as hard as he can manage, but Yamamoto has his head down against his knees and his shoulders trembling with laughter and seriously he is asking for it. Gokudera looks back at the screen, his cheeks starting to burn red and embarrassed, forces himself through another line while the markings of an upcoming duet promises escape. Gokudera can hardly continue alone if Yamamoto won't even sing his part.
But Yamamoto lifts his head in time, grabs at the second microphone, and just as the first word lights up he's singing too, his voice catching into almost-harmony with Gokudera's own. He continues on into his own half of the verse, the lingering laughter audible in his voice, and Gokudera really wants to hit him but he's distracted, he's never heard Yamamoto sing before. It's not that he's any good, really - he can't hold a note steady, and his voice is a little breathy with the traces of amusement at Gokudera's expense - but he sounds delighted, charmed and unselfconscious until Gokudera is caught up in watching the light in his face and nearly misses his cue to join back in. It's not so bad, when he modulates his volume down a little to better fade into Yamamoto's, and neither of them are even close to on-key but Yamamoto is looking up at Gokudera from the floor and smiling like this is all he's ever wanted, and the joy in his face burns off the worst of Gokudera's flush.
It's easier to commit to the next lines when he's looking at Yamamoto, to nearly shout his line like he's handing off a baton to the other boy. Yamamoto has to glance at the screen to check the words but Gokudera has his in his head, can focus his attention on Yamamoto's expression instead, the shine of his smile like he can't keep the happiness off his face. Gokudera snaps his hand down just before the duet "Hey!" to count them off and Yamamoto follows his lead without being told, both of them falling into time with each other if not into tune. It's easier to follow the rhythm of the song, now that Gokudera has heard a verse of it, enough that he can pay attention to what he's actually saying. He can feel himself going hot again, his throat trying to close up on the lyrics that are maybe closer to true than he expected, and he doesn't dare look at Yamamoto's face now even though he can feel the other's eyes on him.
The musical interlude is longer than Gokudera expects, gives him a moment to catch his breath and try to shove aside the embarrassed heat flooding his cheeks. The second verse isn't so bad, and it distracts Yamamoto from his continued consideration of Gokudera's expression, and Gokudera thinks he's fine again until he can see the lyrics "Say it, watch it, I want you to understand" coming at him from the screen like a dare. He frowns at the oncoming threat, but it's not like he's going to stop now. He takes a breath, forms the words on his tongue, and when they come out they're louder than he expected, like the sparkle in Yamamoto's eyes is breathing strength into his voice. He's still off-key, every word falling flat as Yamamoto's own voice shivers into breathiness and laughter alternately, but Gokudera's starting to smile in spite of himself, affection and contagious delight getting the better even of his sustained glare, and Yamamoto's enthusiasm is burning into his throat, pulling his lines into growled intensity that just makes the other boy smile wider.
By the last line Yamamoto is on his feet, Gokudera has abandoned the chair in favor of edging fully into the other's personal space; they're nearly touching, so close Yamamoto lets his microphone fall and sings into Gokudera's to avoid feedback. They make it through the last words smoothly if not gracefully, the mismatch of their voices tangling into something somehow better than either of them alone. There's a metaphor there, Gokudera can see it clear, but he doesn't linger on it, just lets the microphone drop and leans in for the bright draw of Yamamoto's smile. The other boy is laughing again, warm and bubbling with sincerity like his laughter always is, as true and unfeigned as Gokudera's reactions never manage to be, but his mouth goes soft as Gokudera's lips brush his, amusement fading into a sigh of satisfaction as his eyelashes flutter.
Gokudera isn't sure if he's objectively any better at kissing than he is at singing. But Yamamoto is melting under his touch like he's heat and sunlight, opening his mouth to touch his tongue to Gokudera's lower lip, and Gokudera doesn't care what anyone else might think about his technique. His singing makes Yamamoto smile, and his kissing makes Yamamoto sigh, and that's all Gokudera wants to be able to do.