Adding mute and unmute functionality to a Twilio Voice JS SDK application enables real-time control over audio, which is essential for voice calling applications, conference bridges, or other voice-enabled solutions. To implement this feature, developers need to add an HTML button element, retrieve it using JavaScript, and attach a click event listener that toggles the mute state of the call using the `call.mute()` method. The updated state of the call's mute status should be saved and reflected in the mute button's text accordingly. With this functionality, users can control audio in their voice SDK app powered by Twilio Programmable Voice, ensuring a seamless user experience.