Removing sound dampening windows for cleaning

I have sound dampening windows in my new home, they follow the style of the other windows. While most are on tracks and removable, the large picture window in the living room has a stationary middle window.

Unfortunately the two windows (the normal house windows and the secondary sound dampening windows) are quite dirty and need to be cleaned. I cannot figure out how to remove the sound dampening window.

There are brackets at the top, but removing them did nothing. The window still seems very installed or stationary.

Has anyone else encountered this?

Any suggestions?