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@Suiseiseki@freesoftwareextremist.com The song "テトリス" indeed contains the melody of "Коробушка" which is out of copyright.

However I'm pretty sure the song itself is copyrighted even though it is based on a public domain work. If you use a public domain work as part of your original work it still copyrightable (like Disney cartoons based on public domain fairytales) as far as I know.
I'm not sure if the copyright holder of the original character is known.The copyrights to the Kasane Teto character are owned by the ツインドリル doujin circle. The original artist of the character and the voice actor who provided the voice bank are part of this circle.