Ivan has a sear like a necklace around his neck from barbed wires used to choke him, causing him to never remove his scarf in front of anyone unless he really trusts the other.
The injury to his voice box from that incident caused him to retain his high child-like voice that grows quieter as he becomes increasingly upset. The Baltics say that when he is really mad, his voice gets so hoarse and quiet that you're probably not going to hear hear your incoming death.
That is why Ivan prefers to mutter 'kolkolkol' since it is easier to sound out even with a quiet voice.

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a p h ; h e a d c a n o n s
RandomHello! Not much to say, so.. ~•Onto the headcanons!•~ (Finished book as of 11/19/18)