In the absence of an external field, it's zero, because these are eigenstates 9f the Hamiltonian, so, if the electron is in any state of definite n at t=0, it will remain in that state forever.
When the electron jump between orbits in hydrogen atom, its velocity is never constant because either it is accelerating or decelerating. This jump involves change in its energy which is either ejected as photon or absorbed by electron.