Where in the human body is the so called "round window membrane"?

The "round window membrane" is better known as the eardrum. The round window membrane covers the opening to the ear and vibrates which moves the ossicles which amplifies the movement from the drum. The ossicles vibrate the cochlea which resulting waves trigger the hair cells. The hair cells trigger nerve conduction and send signals to the auditory cortex where sound is perceived.

https://en.wikipedia.org/wiki/Round_window