Jack can’t speak but hiccup knows the language. 
I’ve been playing around with a scene for my double Anastasia AU fic and it involves Jack being so enraged he can’t make human words work.
So it had me thinking what if during the north pole fight with pitch damages his vocal cord somehow. Like pitch had just broken his staff and started to strangle Jack, and since he was very weak, the damage stuck. 
At first, Jack would definitely do the same thing as Sandy, but with his frost, but after a while he realizes that he can still make sound.
Jack was trying to force himself to speak like a human, but he’s a winter spirit.
So Jack can communicate through sounds.
Hiccup tried making a guide to what sounds mean what but it’s still a little bit more complex than that. 
Laughter is the sound of silver bells, or the splat of snowballs.
Excitement is the sound of sleds down hills, ice skates gliding.
Worry sounds like Whistling wins, branches breaking, and ice cracking just a little bit.
Fear sounds like the rumble of an avalanche and ice shattering.
Sadness sounds like wolves howling at the moon, The trickling of water from melting snow.
Pain noises sound like icicles being snapped, snowballs filled with rocks, wet gurgling, a car slipping on ice, bones breaking. 
Rage sounds like a howling blizzard, glaciers cracking, pipes bursting. 
Love sounds like animals in their dens and the closest thing jack can get to speaking is the sound of people celebrating.
The guardians still can’t fully understand complex sentences in this new language, but hiccup can.
Hiccup nose by the pitch of ice cracking weather jack means that he is or was worried. Hiccup nose, whether Jack is laughing or saying something was funny by the sound of the different bells.
Hiccups the one to figure out that Jack can make a lot of sounds if they connect even vaguely to winter. Jack can sound like the buzz of Christmas lights or the crackle of a campfire.
Hiccup loves listening to Jack read poetry like this.
Everybody’s name is a different sound as well. 
Bunny- the sounds of rabbits running on snow
North- the sound of reindeer and bells
Sandy- The sound of dried snow shifting
Hiccup has a couple different ones kind of like pet names? The first one was the sound of a snowboard hitting the ground. When Jack learned that he could make more complex noises, he started sounding like a snow wraith and then when he could make more human sounds, he use the sound of someone hiccuping after drinking too much hot chocolate.
To clarify when Jack makes more human noises it’s never his voice, and it’s never the same voice.
Way later down the line he can repeat stories since a lot of people are cooped up inside telling stories during winter. Jack can’t remix the story to speak normally, but it’s still something that he likes doing to entertain children. 
And if we use hiccup is a spirit of bravery, fear whatever he can do the same thing.
They learn each other’s languages and they’re just making noises together.
Hiccup telling Jack how much he loves him through the sounds of heartbeat, leathery wings cutting through air and a blizzard rattling window.
Jack responds with the sounds of silver bell overlapping with children laughing, pages of a story book flipping, The ruffling of blankets and the soft crack of fire ending with a baby hiccup.
And if pitch ever did return, he’ll be met with the sound of 1000 dragons roaring coming from one spirit. 
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