Lola Pons Rodríguez explains a brief history of the letter X, she mentions that it has had several sounds that have changed every few centuries, and she also explains that the letter H was not mute in some regions. It was pronounced in a similar manner to English or German. This video is in Spanish.
(Please remember the ongoing problem that Google/YouTube is not allowing some other browsers to play embedded YouTube videos, which is really bad. Of course, YouTube videos have no problem playing on Google’s Chrome.)