Load content script only for audio files.

app/manifest.json

@@ -8,12 +8,8 @@
       "scripts/background.js"
     ]
   },
-  "content_scripts": [{
-      "matches": ["<all_urls>"],
-      "js": ["scripts/contentscript.js"],
-      "run_at": "document_end"
-  }],
   "permissions": [
-    "activeTab"
+    "<all_urls>",
+    "webRequest"
   ]
 }

app/scripts/background.js

@@ -0,0 +1,21 @@
+async function onHeadersReceived({ responseHeaders, url, tabId }) {
+	const isAudio = responseHeaders.some(header =>
+		header.name === 'Content-Type' && header.value.startsWith('audio/')
+	);
+
+	if (isAudio) {
+		browser.tabs.executeScript(tabId, {
+			runAt: 'document_end',
+			file: '/scripts/contentscript.js',
+		});
+	}
+}
+
+browser.webRequest.onHeadersReceived.addListener(
+	onHeadersReceived,
+	{
+		urls: ['<all_urls>'],
+		types: ['main_frame'],
+	},
+	['responseHeaders'],
+);