block placeholder audio with a short audio clip

ext/fg/js/frame.js

@@ -170,12 +170,27 @@ class Frame {
         }
 
         for (const key in this.audioCache) {
-            this.audioCache[key].pause();
+            const audio = this.audioCache[key];
+            if (audio !== null) {
+                audio.pause();
+            }
         }
 
-        const audio = this.audioCache[url] || new Audio(url);
-        audio.currentTime = 0;
-        audio.play();
+        let audio = this.audioCache[url];
+        if (audio) {
+            audio.currentTime = 0;
+            audio.play();
+        } else {
+            audio = new Audio(url);
+            audio.onloadeddata = () => {
+                if (audio.duration === 5.694694) {
+                    audio = new Audio('mp3/button.mp3');
+                }
+
+                this.audioCache[url] = audio;
+                audio.play();
+            };
+        }
     }
 
     handleError(error) {

ext/manifest.json

@@ -1,7 +1,7 @@
 {
     "manifest_version": 2,
     "name": "Yomichan",
-    "version": "1.0.12",
+    "version": "1.0.13",
 
     "description": "Japanese dictionary with Anki integration",
     "icons": {"16": "img/icon16.png", "48": "img/icon48.png", "128": "img/icon128.png"},
@@ -31,13 +31,14 @@
     ],
     "web_accessible_resources": [
         "fg/css/frame.css",
+        "fg/frame.html",
         "fg/img/add_kanji.png",
         "fg/img/add_term_kana.png",
         "fg/img/add_term_kanji.png",
         "fg/img/play_audio.png",
         "fg/img/spinner.gif",
         "fg/js/frame.js",
-        "fg/frame.html",
+        "fg/mp3/button.mp3",
         "fg/ttf/kanji-stroke-orders.ttf",
         "fg/ttf/vl-gothic-regular.ttf"
     ]