fix(Signaling) Return correct media type for Audio sources

service/RTC/SignalingLayer.ts

 
     const firstLetterOfMediaType = sourceName.substr(firstLetterOfMediaTypeIdx, 1);
 
-    for (const type of Object.values(MediaType)) {
-        if (type.substr(0, 1) === firstLetterOfMediaType) {
-            return type;
-        }
+    if (firstLetterOfMediaType === 'v') {
+        return MediaType.VIDEO;
+    } else if (firstLetterOfMediaType === 'a') {
+        return MediaType.AUDIO;
     }
 
     throw new Error(`Invalid source name: ${sourceName}`);