Pressing F11 toggles full screen.

If the navigation toolbar is hidden rather than being in full screen mode, right-click on an empty space in the tab bar then click on the "Navigation toolbar" entry to select it.

when I press f11 it activates and disactivates the sound What should I do

Are you on a Mac?

Mac: command+Shift+F

I have an hp and I dont know what the command button looks like

Press Fn + F11

thank you so much