See https://gist.github.com/brettz9/0cfda9e04751090f4a7a for an example use on a particular Mediawiki installation (not abstracted to an extension, but could easily be so)

chunkify.js

/**
 * Chunkify
 * Google Chrome Speech Synthesis Chunking Pattern
 * Fixes inconsistencies with speaking long texts in speechUtterance objects 
 * Licensed under the MIT License
 *
 * Peter Woolley and Brett Zamir
 */
 
var speechUtteranceChunker = function (utt, settings, callback) {
    settings = settings || {};
    var newUtt;
    var txt = (settings && settings.offset !== undefined ? utt.text.substring(settings.offset) : utt.text);
    if (utt.voice && utt.voice.voiceURI === 'native') { // Not part of the spec
        newUtt = utt;
        newUtt.text = txt;
        newUtt.addEventListener('end', function () {
            if (speechUtteranceChunker.cancel) {
                speechUtteranceChunker.cancel = false;
            }
            if (callback !== undefined) {
                callback();
            }
        });
    }
    else {
        var chunkLength = (settings && settings.chunkLength) || 160;
        var pattRegex = new RegExp('^[\\s\\S]{' + Math.floor(chunkLength / 2) + ',' + chunkLength + '}[.!?,]{1}|^[\\s\\S]{1,' + chunkLength + '}$|^[\\s\\S]{1,' + chunkLength + '} ');
        var chunkArr = txt.match(pattRegex);
 
        if (chunkArr[0] === undefined || chunkArr[0].length <= 2) {
            //call once all text has been spoken...
            if (callback !== undefined) {
                callback();
            }
            return;
        }
        var chunk = chunkArr[0];
        newUtt = new SpeechSynthesisUtterance(chunk);
        var x;
        for (x in utt) {
            if (utt.hasOwnProperty(x) && x !== 'text') {
                newUtt[x] = utt[x];
            }
        }
        newUtt.addEventListener('end', function () {
            if (speechUtteranceChunker.cancel) {
                speechUtteranceChunker.cancel = false;
                return;
            }
            settings.offset = settings.offset || 0;
            settings.offset += chunk.length - 1;
            speechUtteranceChunker(utt, settings, callback);
        });
    }
 
    if (settings.modifier) {
        settings.modifier(newUtt);
    }
    console.log(newUtt); //IMPORTANT!! Do not remove: Logging the object out fixes some onend firing issues.
    //placing the speak invocation inside a callback fixes ordering and onend issues.
    setTimeout(function () {
        speechSynthesis.speak(newUtt);
    }, 0);
};