rebase + add audio desc

Author: fippo

index.bs

@@ -362,7 +362,7 @@ dictionary RTCEncodedVideoFrameMetadata {
     </dt>
     <dd>
         <p>
-            The codec MIME media type/subtype. e.g., video/vp8 or equivalent. 
+            The codec MIME media type/subtype. e.g., video/vp8 or equivalent.
         </p>
     </dd>
     <dt>
@@ -468,6 +468,14 @@ dictionary RTCEncodedAudioFrameMetadata {
             that is used to describe the format of the RTP payload.
         </p>
     </dd>
+    <dt>
+        <dfn>mimeType</dfn> of type <span class="idlMemberType">DOMString</span>
+    </dt>
+    <dd>
+        <p>
+            The codec MIME media type/subtype. e.g., audio/opus or equivalent.
+        </p>
+    </dd>
     <dt>
         <dfn>contributingSources</dfn> of type <span class=
             "idlMemberType">sequence&lt;unsigned long&gt;</span>