There is always music in the TARDIS. She is vast and complex and exquisitely alive, and every part of her thinks and feels and has a voice. Her engines thunder in bass drum rolls and hum in deep cello tones. Her monitors and instruments sparkle in flute trills. A thousand other systems hit every note in between, some of them speaking not through sound but through light and heat or a ripple in the air. She is an orchestra all her own, and every flight is a symphony.
It's not often that the TARDIS deliberately composes. Most of her singing is simply the work of the moment, the everyday sounds of flight and pursuit and life made musical by the harmony of a thousand voices with a single goal. Not simple melodies but clear ones, and so familiar that sometimes she catches her pilot singing along under his breath. The music of everyday wonder.
Tonight is different. There is always music, but tonight there is a song.
It has been a day of celebration for everyone, for her pilot whom she loves as much as she loves the stars, for the humans who love each other just as fiercely, for a universe that was unmade and rebuilt into rightness. They have all danced and lifted their voices, and now the TARDIS has her turn. Free, unfettered by danger or duty, she sings a song of joy and triumph to every corner of time and space.
She steers herself on a course of her own design, one that turns the trumpet crescendos of her navigational systems into fanfares every time she spins and turns. She's dancing among her beloved stars, and the dance is its own song. To them she sings the delight of finding a long-lost friend again, here in this remade universe where there have always been stars. They are here again and so is she, freshly reborn from a memory and free to embrace them again. They echo back her sounds of love with confusion in their crystal voices, not remembering that they were lost, not remembering that she was lost. Her fanfares soften in affection, and underneath them the lower brass notes sing a playful warning: Be prepared. I'm on my way.
Inside, the song has the same heart but in different tones. Even gentled, her hymn to the universe is a violent bellow compared to the peaceful refrain that drifts down her corridors. All is well, her climate controls whisper in piano and clarinet shades. Her human passengers are always part of the song, even asleep as they are at the moment, and they are also part of its singing as she regulates her temperature and atmosphere to suit their comparatively fragile systems. The TARDIS sings gentleness to them, a lullaby of love and gratitude that mirrors their own soft breathing and tranquil heartbeats. You have done great things, you small and amazing creatures. Rest well.
The one whose song holds the entire universe in its laughing trills smiles in her sleep as the melody washes over her. The other one, who sings in resonant chords that are vastly older than his body, wraps his arm around her more tightly and pulls her closer. Neither of them hears the song, far more attuned to each other than to the TARDIS even in sleep, but they can feel it.
Someone hears. The TARDIS lets the most complicated part of her song follow her pilot to the library, where he wanders the stacks in solitude. So often lonely, but tonight just alone. All his rhythms speak of comfort and contentment, the peace of recognizing the rightness of the universe. She knows his song almost as well as she knows her own, its depth and complexity familiar and constant even if the voice changes now and then. And he recognizes her voice, hears and feels and understands it in ways that no human and no star could ever master. He feels the thrum of her engines singing to themselves, hears her fanfare to the stars, breathes in her lullaby to their human charges. And over it the TARDIS sings a song that's just for him, a love song in violin patterns of changing light and bursts of warmth. He's a part of the singing as well, and he is always, always the reason for the song.
On other nights, he would be adding his own notes to her song, bouncing around her control panel, spinning a dial here and tweaking a lever there, shifting the melody into something that was half her voice and half his. They sing well together, but she appreciates his hands-off approach tonight. This is her song, and she recognizes his respect for that in the quiet way he moves with her, following the melody and letting the undertones push him along. She speaks to him of the wonder of the universe, how it is more wonderful for having such things as adventurers in it, and childhood dreams to keep them alive, and time machines to take them everywhere and everywhen. She sings of places they've seen and places they have yet to see, and through it all there is the eager, fluttering refrain that she wants to see it all with him, and wants him to see it all with her.
The end of the song is the soft blaze of a single table lamp, one perfect piccolo note skimming the melody like a sea bird skimming the water's surface. The song fades but the music remains. Her pilot touches that last light, quietly radiating love and praise. "Beautiful, as always," he murmurs. Her contentment is a ripple of harp strings that makes him laugh, which is its own kind of music. He offers a song of his own, using his voice rather than manipulating hers, overlaying his melody on her music not to upstage her, but to repay her in kind. It's a human song, not one of hers, but she loves it anyway for the way it answers her own song.
"Fly me to the moon, and let me play among the stars…"