paste this in the browser console then drag an audio file to the page to replace the notification sound

replaceWhatsappWebNotificationSound.js

(async function(){
	async function writeSound(cache, url){
		const el = document.createElement("div");
		document.body.appendChild(el);
		el.style.background = "white";
		el.style.width = el.style.height = "100%";
		el.style.position = "absolute";
		el.style.zIndex = 1000;
		el.textContent = "drag an audio file to this page";
		el.addEventListener("dragover", e => e.preventDefault());
		const file = await new Promise(resolve => {
			el.addEventListener("drop", e => {
				resolve(e.dataTransfer.files[0]);
			});
		});
		const response = new Response(file);
		cache.put(url, response);
		el.textContent = "refresh the page to apply the changes";
	}

	const cacheKeys = await caches.keys();
	for(const cacheKey of cacheKeys){
		const cache = await caches.open(cacheKey);
		const keys = await cache.keys();
		for(const request of keys){
			if(/https?:\/\/web\.whatsapp\.com\/notification.*\.mp3/.exec(request.url)){
				await writeSound(cache, request.url)
			}
		}
	}
})();