Hello readers!
This is a little headcanon that I've had for a while, so I decided to finally put it into writing. I don't have a lot of experience writing fiction, and this is actually the first fanfic that I've ever shared! So constructive criticism is greatly appreciated. Hope you enjoy!

Mikey barely heard it, making his way to the kitchen after training that morning. He paused and took a few steps backward, near the bathroom where he could hear the shower running. Very soft, through the sound of the water hitting the tile floor, he could hear a faint and familiar humming. He stopped and leaned against the wall with a smile on his face, listening carefully through the falling water and the usual hum of the lair. Leo spotted him from across the pit, on his way to the kitchen as well, and gave him a questioning look. Mikey held a finger to his smiling lips and nodded towards the bathroom door. His older brother nodded with understanding and continued on his way.

They had come to a sort of unspoken agreement, the three of them. The first time they had heard Donatello sing when they were much younger, they had all brought it to attention, mostly in praise —excluding Raphael at the time, whose eight-year-old self had deemed it "lame"— and their brother had never looked more embarrassed, obviously not intending for his family to hear. And they didn't hear him sing again, for a very long time. Months later, when the three of them came by Donatello's room to ask him to be a part of their game of hide and seek, they could hear soft words coming from behind his bedroom door, a song from an old animated movie they had watched the night before. The three brothers stopped and listened for a few moments, as Donatello sang to himself and puttered round his room, before looking to each other in a silent understanding, and decided to make their game a three-person one for now. When their brother emerged from his room not long after, no one said a word about what they had heard, all of them remembering what had happened last time.

They didn't want him to stop again, because he was good — very good, for his age. And his voice only improved as he got older. (Though there was a period of time, as all of the boys were going through puberty, with his voice fluctuating so much, that no one heard Donnie so much as hum. Each of them was grateful when they heard him start singing again, his now lower voice even better than it was before.)

He continued to hum or sing when he thought his family couldn't hear him, while he worked or did his chores, or while he was in the shower. And they all loved hearing it, how comfortable he sounded. He just sounds so happy, Mikey thought, standing outside of the bathroom and thinking back to all of the other times he'd heard his reserved brother pick up a tune. Relaxed. Maybe that was a better word for it. It was something that his family was more than happy to hear, a drastic shift from his normally anxious and stressed demeanor. Hearing the shower shut off, along with the soft voice, Mikey crept away from the door and continued to the kitchen.

When Donnie came in a few minutes later, Mikey held out his brother's regular morning coffee in one of the family's chipped mugs.

"Mornin' D! Have a good shower?"

"Uh," Don looked around quizzically at his family and then to his little brother, hesitantly taking the mug. "Yeah, I guess… thanks."

Donatello sat down at one of the benches, the confused look plastered on his face until the taste of the coffee seemed to make him forget it.

A few days later, Michelangelo and Raphael were sitting on the floor of the pit when they heard the familiar sounds of the creaky turnstiles and their friend's cheery voice.

"Hey, guys!" April called, making her way towards the two brothers, who were currently playing a racing game. "Uh, sorry to come over unannounced, but I could really use some help with this chemistry project," she explained, shrugging her bag up higher onto her shoulder. "My group is being absolutely no help at all, and my teacher hasn't even taught us-"

"HA, how's it feel to come in second, Raph?"

"Oh come on, quit cheating, you little twerp!"

"How-Ouch!-how was that cheating? You're just a sore loser."

April crossed her arms over chest and called out over their bickering.

"Guys! Is Don around?"

"Oh, uh, yeah," Raph said, eyes never leaving the television screen. "Prob'ly in his lab or somethin'."
"Right. Thanks."

Probably should have just gone straight there in the first place April thought glumly. She turned and started to make her way towards the lab, and could now see that the doors were slightly cracked open, the edges just barely resting upon one another. As she raised her hand to knock on one of the tall metal doors, she paused at the muffled melodies she heard coming from inside. It wasn't the music itself that made her hesitate, she liked listening to music while she worked too, but what she heard over the music itself. Blending together with the recording—coming from a phone or computer speaker, she couldn't tell which—there was singing, murmured so quietly that she missed a word or two here and there, but it was unmistakably Donatello. She lost bits of it to the volume of the music, but she could make out the sound of his voice: soft, but steady and smooth. Comfortable.

...on Jupiter and Mars...
...hold my hand... in other words...
...Fill my heart with song, and let me sing forevermore...
...all I long for, all I worship, and adore...
...In other words, please be true...
...In other words... love you...

April could tell he was moving further back into his lab, only hearing the strongest parts of his vocals as the chorus came back around again.

...Fill my heart with song...
...forever more...

...please be true...
...in other words…

April turned back towards the brothers in the pit. "Hey, I didn't know Donnie could-!" She was abruptly cut off by the two of them shushing her and violently shaking their heads. She took a few steps towards them in confusion.

"We don't talk about it," Mikey informed her, in a hushed tone for once, setting down his remote and bounding up onto the couch so he could be closer to her. "He gets embarrassed," He said with a little smile, as if to say 'how cute is that,' like they were talking about a small child.

"And you especially don't say anything," Raph ordered, pointing at her and narrowing his eyes slightly, going into protective-older-brother mode, before chuckling and leaning back against the couch. "I don't think he'd ever recover."

"Wh-"

"Yeah, no way." Mikey agrees, shaking his head and cutting off her questions. He sits back, relaxing more on the couch. "Don's always done that since we were kids, when he thinks no one is listening." Glancing up at the lab doors he drops his volume slightly more. "If anyone says anything about it he'll stop. Like, stop-stop. For a really long time." April thinks Mikey looks almost sad for a moment, then perks back up. "So we just pretend we can't hear him." He finishes with a little grin and a wink.
April nods with a hum and turns back towards the lab doors.

The song is over now, along with it Donnie's voice, so April knocks lightly on the door before the next one can really pick up.

"Hey, Donnie!"

She hears crashes from inside, and the screech of a heavy table moving as someone bumps into it, then the click of a computer key, stopping the next song.

"Um, heh, h-hey, April! Come on in!"

April let herself in through the doors. As she closed them quietly behind her, she caught sight of Mikey and Raph giving her pleading and threatening looks, respectively. She gave them a reassuring smile and shut the door. She then turned to Donatello and plopped her bag down on the desk, pulling a rubric out of a folder.

"Thanks, Don. So I was really hoping you could help me out with this chem project…"

Thank you for reading!
If you don't know, the song Don was singing is Fly Me to the Moon by Frank Sinatra (my personal favorite, and one I think Don would like). I plan to post more little one-shots and maybe eventually some multi-chapter stuff as I get more practice with writing. I've got a few things in the works right now, but one of the things I struggle with most when writing fiction is coming up with ideas, so if you guys have any headcanons or plot bunnies you'd like to share, I'd love to hear some of them! Constructive criticism is appreciated! Have a lovely day!