A type system is sound when:

    for all expressions e:
      if e type checks with type t, then one of the following holds:
        - e evaluates to a value v of type t; or
        - e does not halt; or
        - e hits an "unavoidable error"
          like division by 0 or null deref
          (what counts as "unavoidable" varies from lang to lang)

Notice anything interesting about this definition? It uses the word "evaluate"! Type soundness is not just a statement about the type checker. It relates type checking to run time (and thus to compilation too). That is, if you muck with Java's runtime or compiler, you can break type soundness, even if you don't change its type system in the slightest.