Trigger event when a css class appear

Question

I've a problem coding an addons for firefox browser. The addons is supposed to make sound when a CSS class appear "error-class" or "warn-class". I use the mutation observer to check if my class appear and to store the fact that I played the sound.

It's for a Grafana dashboard using status panel to alert when a service doesn't respond on time.

here my actual code :

let target = document.querySelectorAll(".panel-container");
let histo = new Array();
for (let i = 0; i < target.length; i++) {
let observer = new MutationObserver(function(mutations) {
    mutations.forEach(function(mutation) {
        let foo = mutation.target.getAttribute("class")
        let error_class = "error-state"
        let warning_class = "warn-state"
        let ok_class = "ok-state"
        let id =   mutation.target.parentElement.parentElement.parentElement.parentElement.parentElement.id;
        if (((foo.includes(error_class))|| (foo.includes(warning_class)))&& !(histo.includes(id))){
          histo.push(id);
          beep();
        }
        if (foo.includes(ok_class) && histo.includes(id)){
          histo = arrayRemove(histo, id);
        }
        console.error(histo);
    });
});

// configuration of the observer
let config = { attributes: true };

// pass in the target node, as well as the observer options
observer.observe(target[i], config);
}


function arrayRemove(arr, value) {
 return arr.filter(function(ele){
    return ele != value;
 });
 }

function beep() {
    var snd = new Audio(<audio in base 64>)

and here the manifest :

{
  "manifest_version": 2,
  "name": "sound_on_grafana",
  "version": "1.1",

  "description": "make sound for error and warning state",

  "icons": {
    "48": "icons/border-48.png"
  },

  "content_scripts": [
    {
      "matches": ["<all_urls>"],
      "js": ["sound_on_grafana.js"]
    }
  ]

}

the expecting result is when "error-class" appear on an element of the html page the sound in base64 is played