Helper to watch for DOM elements getting added and saying the contents.

say_last_child.js

function sayLastChild(selector) {
    // Select the node that will be observed for mutations
    const targetNode = document.querySelector(selector);

    // Options for the observer (which mutations to observe)
    const config = { attributes: true, childList: true, subtree: true };

    // Callback function to execute when mutations are observed
    const callback = function(mutationsList, observer) {
        // Use traditional 'for loops' for IE 11
        let mutation = mutationsList.pop();
        if (mutation.type === 'childList') {
            console.log("saying:", mutation.target.innerText);
            window.setTimeout(() => {
                let ut = new SpeechSynthesisUtterance(mutation.target.innerText);
                speechSynthesis.speak(ut);
            }, 0);
        }
    };

    // Create an observer instance linked to the callback function
    const observer = new MutationObserver(callback);

    // Start observing the target node for configured mutations
    observer.observe(targetNode, config);
}

Example delaying the mutation observation for 2 seconds and matching class starting with something:

(function() {
    'use strict';
    window.setTimeout(() => {
        sayLastChild('div[class^="styles_messageList"]');
    }, 2000);
})();