Hint for first part

templates/game/index.html.twig

@@ -15,10 +15,9 @@
     <div id="game-container">
         <div id="mercure-config"
              data-mercure-public-url="{{ mercure_public_url|e('html_attr') }}"
-             data-topic="{{ (mercure_topic_base ~ '/game/hub')|e('html_attr') }}"
+             data-topic="{{ (mercure_topic_base ~ '/game/hub-' ~ session.id)|e('html_attr') }}"
              data-api-ping-url="{{ path('game_api_ping')|e('html_attr') }}"
              data-api-echo-url="{{ path('game_api_message')|e('html_attr') }}"
-             data-user-id="{{ user_id|e('html_attr') }}"
              style="display:none">
         </div>
 
@@ -28,6 +27,9 @@
         <div id="message-container">
 
         </div>
+        <audio id="message-sound" style="display:none">
+            <source src="{{ asset('audio/message.mp3') }}" type="audio/mpeg">
+        </audio>
         <div id="input">
             <input type="text" disabled id="input-message">
         </div>