That's not what soundness means in a type system. Of course untyped input has to be parsed and validated.

An unsound type system means that you can write code that compiles, but experiences a runtime error caused specifically by the types (not the values or the business logic conditions) not actually being compatible. The famous example is having an array of a subtype being used as an array of the supertype:

function doStuff(animals: Animal[]) { animals.clear() animals.push(new Cat()) }

const dogs: Dog[] = [new Dog()]

doStuff(dogs)

dogs[0].bark() // <-- type error

In a sound type system, that code would not compile because we know that you can't treat an array of Dog as an array of Animal in general.