It's never been about the physical stuff for Donnie, not really. Sure, he likes the weight of his brothers' bodies in his arms from time to time, a kiss on the lips or cheek, but anything more than that feels uninteresting, even uncomfortable if he gets too far. It took a while for him to put that into words for his brothers, but they understood  
Just because he doesn’t want to get physically involved, though, doesn’t mean he doesn’t have a place in their bedroom, even if it’s off to the side. The particular nature of his attraction might not care for touch, but his other senses are all happy to work overdrive on the nights they come together.  
Seeing it is good--great, even. If he was the poetry kind, Donnie would write about the movement of those three forms intertwined, gleefully putting on a show for him or simply welcoming him into their love. Whether they're drifting through a dim haze or shining in warm light, his brothers are so fucking beautiful it takes his breath away  
And of course, the smells are wonderful. They all have their own kinds of musk, and it’s only magnified by stuff like anger or pleasure, until it’s something you could almost get high on. Add the raw scent of sex and it’s just this side of overwhelming; Donnie relishes every time he catches a whiff of it on his brothers in the aftermath.  
The  sounds,  though, are what strike deep in his heart. Sighs, whispers, breathy moans, the creak of the bed, giggles and rustles, squelches that are the perfect kind of gross, the slap of flesh again flesh. Even the squeaks and churrs, their more animal sounds popping through, are comforting, because that reminds Donnie it’s his family he’s listening to.  
Raph's bass tone, Mikey's silver laugh, Leo's voice swirling up and down the scale between them: together, they form an exquisite chorus. Alone, they’re pitch-perfect solos, each charged with their own kind of power. It’s hot, he supposes, really hot, but more than that, it’s  beautiful.  
A song, really, that rises into a crescendo, crests, breaks, only to come crashing softly down again. Their bodies are their instruments and Donnie will never get enough about how well they play for him and for each other, how hard they work at getting things right while still clearly having so much fun.  
And the fact that they let him join in, in his own way, feels like a gift (even though they’ve reminded him that it shouldn’t, that he’s just as worthy of all this as they are, that  he’s  the gift as much as they are). The idea of simply being able to enjoy his own moans and sighs, to feel as sexy as they are without even having to touch anyone, it makes him feel warm in a way that’s a far cry from all the cold, anxious nights he spends in his lab, try to make things that will keep them safe.  
Donnie watches them, listens to them, taps his tail and touches his cock to their beat, and he  knows  they’re safe, knows that they’re happy. It’s a feeling of absolute certainty they don’t get nearly as often as they should, and the feeling elevates the sounds they make from simple pleasure to celebration, a victory chant and a love ballad and a cry of defiance all wrapped up into one.  
"It's like music," he confesses, once, in the post-orgasm haze where you can say weird shit like that without feeling stupid. “Like a song, you know? Our own kind of song.”  
He’s not sure if they understand, really, at least not until he gets the tape for a mutation day present. At first, he’s caught off guard by the sounds that issue from it: lewd, loud rumbles that make him blush as they echo through the living room. Raph and Mikey giggle, the bastards, while Leo sighs and shakes his head, like Donnie can’t see the teasing smile on his lips.  
“Enjoy your mixtape, bro,” Mikey croons, cackling when Donnie reaches over to smack on the arm. Donnie tries to look pissed-off, but he can’t help it–he’s warmed by the knowledge that they made this for me, no doubt while he was wrapped up in yet another project he really should have let them drag him away from, to be honest.  
They’d overcome all their insecurities, Leo’s discomfort with his squeaking and Raph’s worries about whimpering “like a girl” and even Mikey’s occasional concerns that they’d giggle too much, and they’d taken their voices and given them as a gift to Donnie, because they knew how much they’d loved them–because they valued his voice so much, in turn. They’d made it so no matter what happened, he’d have a piece of their song kept close to his heart for them to enjoy.  
And he does, is the thing. On the nights when they’re fighting with each other, when Leo won’t stop training or Raph is out with Casey or Mikey doesn’t leave his room or even Donnie himself can’t find the strength to emerge from his lab, he’ll plug the headphones in and turn the sound on. The voices of his brothers, willingingly offered to him, echo through his ears, building in tune with their pleasure.  
He jacks off, of course, but the comfort also touches him on a deeper level, this visceral reminder of the strength behind the love they share, the compassion and creativity. He’ll play it through as many times as he needs, sometimes pleasuring himself physically, sometimes just sitting there and listening.  
And when he’s ready, he’ll carry the music in the back of his mind as the inspiration he needs to carefully make his way out of the lab. The sounds will give him strength as he bullies Leo into putting his sword down, visits Mikey in his room, sends a firm text calling Raph home. As he pulls his family together and sets them making new songs together.