I am not sure I completely understand what your goal is. Do you want to detect the location of a source inside a metal case from the outside? In this case, you will not have simple air-borne sound from source to receiver. You will have to take into account and model the excitation and vibrations and radiation of the metal case itself, which can become very complicated very quickly. If you want to detect the location from the inside, you still have to take into account the metal case, since you will have a lot of reflections / modes, and possibly structure-borne sound in the metal case as well.