To insert a silent portion of a given length, use the Generate Silence function from the Effects menu. To silence a portion of a sound, first select the portion you wish to silence and then choose Amplify from the Effects menu, with a setting of 0%. (Make sure that % is selected and not dB!) This has the advantage that you can optionally choose a short fading, which will make for a smooth transition to the silent portion instead of an abrupt transition which may sound like a crack.