Zooming is done with the Zoom toolbar buttons 
Click the button. The button remains active until you press it again. The cursor changes into a magnifying glass. Use your mouse to select a sound segment. The selected segment will be expanded on screen so that it will fill the complete graphical window. Click on the same button again to stop using the zoom-in cursor. The positioning tool above the graph shows the relative position of the currently zoomed part (in white) with respect to the complete file.
Click the button. The button remains active until you press it again. The cursor changes into a magnifying glass. Use your mouse to select a screen section that will contain what currently fills the complete graphical window. Click the same button again to stop zooming out.
Click the button and the complete sound file will be displayed in the graph window (no zoom factor).