To run silently, simply make sure the "Mute All Sound Effects" (on the Options customization page) is ON. Note, however, that you will not hear the handy New Mail vocal alerts feature if you choose this option.

You can use the four Sound Group checkboxes to mute groups of sound effects. The groups are: New Mail and Keyword Alerts, Pane "swoosh" and Mark-For-Later effects, Headline Group announcements, and Browsing and Searching noises.

You can have finer control of the sounds, in that you can mute/unmute INDIVIUDUAL sound effects, or assign the sound .WAV file of your choice to the sound events.

To change effects, start by pushing the Change Sounds... button, or from the Windows Control Panel, open Sounds.

Scroll down through the event list until you reach WorldFlash Sounds. Here will be a list of the individual sound effects, allowing you to select sounds and then choose a waveform file (.WAV) to play. Use the Browse... button to find a sound file, and the Play icon in the Preview area to listen to it. You can also choose "(None)" as a sound file to mute that specific sound event.