Hiccup always thinks about it, the once "mindless beasts" having an amazing capacity to elevate their roars to higher pitches. He lies in bed, leg and stump stretched out comfortably, listening to what could only be called "singing". Toothless slinks out in the evening and no amount of calling from his master and friend can bring him back. Hiccup gives it up after a while and is content to just lie in bed, content in the knowledge that his best friend will never abandon him.
The Terrible Terrors have the make the highest pitch, and there are a great many of them. The group of them together sounds like a choir of birds. Though perhaps, Hiccup thinks with a smile, with more scales than feathers.
The Gronkles have a deep voice, and let a low rumble release at regular intervals. This is interloped with the Terrors' shrill squeaking.
The Monstrous Nightmares have an odd, but not unpleasant, voice. They lift their heads to the sky, bodies aflame, and roar a vibrating note that shakes the ground and makes the spine of any human lucky enough to listen, shiver.
The Deadly Nadder makes a surprisingly shrill noise (though not as high as the Terrors'), given a hollow quality by the roundness of the beast's mouth. It lets the sound reverberate before releasing it to mingle with the voices of its fellow dragons.
The Hideous Zippleback hits both ends of the alto scale thanks to its two heads. Each head lets out a note that matches perfectly with the other. This ability gives it one of the nicest voices in all of dragonkind. Yet the heads cannot hold a candle to the master of them all.
The Night Fury has a voice that sounds as though it comes from the stars themselves. It starts low, and rises to the highest point on the scale. He circles above the others, invisible to all, the majestic roar coming from all directions.
The dragons do not do this often, now that Hiccup reflects on it, not more than once a year. It is always at the peak of Berk's very brief summer. Why they do it, he knows not. Perhaps it is some draconic custom he is unaware of. The dragon tamer rolls over, making a note to ask Toothless about this later. Not that the Night Fury can actually reply, but the human and his friend manage to communicate rather well, despite the language barriers.
The brunette boy rolls back on to his back, but does not go to sleep. He lies awake and he listens, enchanted and filled with delight. To songs he can hear, only one night a year. The dragons are singing tonight.