Earth is like a jaffacake.
Its flat, circular, has a gooey orange middle and a thin layer on the top. Why can't the chocolate layer be in the form of plates on a flat earth, moving over a delicious Jaffa mantle?
Why do you have to have a ball to have seismic waves? Imagine earth to be a bit like a tuning fork. You can have lots of shape and mass combinations and still ring the same note.