First, make sure that the sound is not highlighted initially. To do this, you must click under the sound (the white area of the screen) and drag your mouse to select the part you want to cut. Press "Ctrl+C" or select "Copy" from the "Edit" menu. To Paste the portion of a sound, press "Ctrl+V" or select "Paste" from the "Edit" menu. Ctrl+X or "Cut" will work as expected.