"Doctor? Does the TARDIS sing? I heard music... but it was almost like it was in my head."
The Doctor looked up at his current companion, a young girl about twenty-two years old who had been traveling with him for a few months.
So soon? She's only been here for three months... he thought sadly to himself. He passed a hand over his face and then mustered a smile (albeit a sad one) as he turned to face her.
"Yes... sometimes... she does."
He had first noticed it when he was newly regenerated into his ninth form, in all his angst-y, black leather jacket-ness. Distraught and mourning for his people and many others at the end of the last great Time War. It was after the TARDIS had changed her desktop.
On the day he met Rose Tyler, when he returned to the TARDIS after saving her from the shop dummies and blowing up her shop... he heard it for the first time. It was very soft at first, he was surprised he had even noticed it. It was a beautiful but mournful sound: lilting and haunting at the same time. It was almost as if... as if the TARDIS herself were mourning for Gallifrey, the other Timelords, and even himself. It was then that he learned for sure that the soul of the TARDIS was female because the voice was definitely a woman's, and it sang its wordless melody over the usual hum heard inside the console room. He found that the singing continued until he went back for Rose; then the normal, comforting hum of the TARDIS returned.
But it didn't stop for good, because every once in a while he would hear that melody again, when faced with a difficult decision or reality... never truly aloud, but in his mind. He was sure Rose could not hear it, but after the Bad Wolf incident, when she had all of the Time Vortex in her mind, he could hear it. When he kissed her, absorbing the time energy into himself, he could hear that song again. So could Rose from then on.
And then he heard it again with his tenth face, but it was stronger. It was no longer a single voice, but like a whole choir. And so much more hopeful. His life had meaning. Rose had taught him how to live with himself again. She had brought back his humanity (even if he technically wasn't human).
Then the TARDIS sang for Rose, mourning for her though she was not dead. It was the same voice he had heard that first time. A single, woman's voice. He realized then that the TARDIS had actually sung for both of them that first day that they'd met.
He was glad that when the TARDIS sang for Martha it was a different song, a different voice. This one was a deeper voice, still a woman's but deeper, and her song grew the longer she was with the Doctor. The stronger she became, the stronger her song became until the day that she left. The Doctor would never have admitted it, but in his deepest hearts he was so proud of her for leaving... for understanding and knowing what she wanted with her life and knowing that she had helped him to cope with losing Rose. Yes, Martha's song fit her perfectly: strong, confident, not forceful, but full of presence nonetheless. The healer. He knew that she would make a good Doctor some day. He wasn't surprised when she called him back to Earth during the Sontaran invasion to find that she had finished her medical training to become a doctor.
Then there was Donna. When he first brought her aboard for good (not after her wedding, after the Adipose incident), he did not hear the singing. He took this as a good sign. Maybe this time it will work. She's my best mate, and maybe she will travel with me forever... He could never have guessed what he would have to do to her one day.
The Doctor heard the TARDIS sing for Donna when he realized he would have to erase her memory. It was like a choir again, a strong, full sound. It was perfect for Donna, and he knew she could hear it, even if she would never remember it. More than ever, he wished at that moment that the TARDIS had never sung.
He thought vaguely that it might stop after he regenerated again, especially when the Old Girl changed her desktop again, but she sang once again for him. But this was different from anything he had heard yet. It was another choir, but in addition to being powerful... it was... reverent? And hopeful... more hopeful than he'd ever heard it. It sounded like the Old Girl (Sexy thing), was proud of him. But then of course he had to screw up and come back two years later for Amelia (still better than Amy, more fairytale...) Pond. Making her wait... again. When he brought her inside, the TARDIS immediately began to sing, but he wasn't worried. The song it sang for her was full of wonder... a pure, innocent, childish wonder that had come from her lifetime of dreaming about her Raggedy Doctor. No, her song did not worry him then. In fact, he could not remember ever being so happy at hearing the TARDIS sing as he was that night when he first took Amy away.
But her song changed when they were on Starship UK. It was a different voice from any he had heard before, and the song, while still wondrous, was a bit sadder. He refused to believe that the TARDIS would be warning him of her departure so early though, and he found that the TARDIS sang for Amy quite often... which in turn led to the Old Girl singing to River just as much. Even to the very end for both of them.
River. He was finished with their timeline. He had taken her to the Singing Towers of Darillium, and they had sung... but little did River or anyone else know that they sang for her because of the TARDIS. It was a bit of a paradox... the Singing Towers only actually sang once, and they sang for River Song. The Doctor knew that the TARDIS was telling him their time together was up. And he cried. The worst part was he knew that River understood that. Being the Child of the TARDIS, she could always hear the singing.
The TARDIS had sung for all of his other companions since then. He had learned to dread it in a way because he knew it meant the end of his time with them was near. His current companion was the third one after Amy and Rory... and now the TARDIS was singing for her too.
He was jolted from his memories when she spoke again.
"It was beautiful, but... kind of sad. Does the TARDIS sing often?"
He stared at her for a moment trying to conceal his own sorrow. After a few moments he shook his head and answered truthfully:
"...Only when she cries."