Add clarifications to audioLevel (#255)

* Add clarifications to audioLevel

* Address review comment

* minor change in refs

* Apply suggestions from code review

Co-authored-by: youennf <youennf@users.noreply.github.com>
Co-authored-by: Jan-Ivar Bruaroey <jan-ivar@users.noreply.github.com>

---------

Co-authored-by: Guido Urdaneta <guidou@chromium.org>
Co-authored-by: youennf <youennf@users.noreply.github.com>
Co-authored-by: Jan-Ivar Bruaroey <jan-ivar@users.noreply.github.com>

Author: 4 people

index.bs

@@ -708,6 +708,23 @@ dictionary RTCEncodedAudioFrameMetadata : RTCEncodedFrameMetadata {
             The audio level of this frame. The value is between 0..1 (linear), 
             where 1.0 represents 0 dBov, 0 represents silence, and 0.5 represents
             approximately 6 dBSPL change in the sound pressure level from 0 dBov.
+
+            If the frame comes from a remotely sourced track, this MUST be
+            converted from the level value defined in [[!RFC6464]]. If the 
+            [[!RFC6464]] header extension is not present in the received packets of the frame,
+            this value MUST be absent.
+            This RFC defines the audio level as an integral value from 0 to 127
+            representing the audio level in negative decibels relative to the
+            loudest signal that the system could possibly encode. Thus, 0
+            represents the loudest signal the system could possibly encode,
+            and 127 represents silence. To convert these values to the linear
+            0..1 range, a value of 127 is converted to 0, and all other values
+            are converted using the equation:
+            <code class="math">10^(-rfc_level/20)</code>.
+
+            If the frame comes from a locally sourced track, the level MUST be
+            taken directly from the source and used as input to generate a value
+            for the [[!RFC6464]] header extension, if negotiated.
         </p>
     </dd>
 </dl>