It looks like this is available in native layer ready for plumbing: https://cs.chromium.org/chromium/src/third_party/webrtc/audio/channel_receive.cc?sq=package:chromium&dr=CSs&g=0&l=593

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/86c8a3c5f1fee11f65b22897ef822cda7750ce23

commit 86c8a3c5f1fee11f65b22897ef822cda7750ce23
Author: Henrik Boström <hbos@chromium.org>
Date: Fri Jan 11 10:39:17 2019

Expose RTCRtpContributingSource.audioLevel.

This is a member in both RTCRtpContributingSource and the derived
dictionary RTCRtpSynchronizationSource. For SSRCs, this value must
always be present, either from the RFC6464 header extension data or
obtained through calculations, though the third_party/webrtc
implementation only does the former. For CSRCs, this value is optional
depending on if the RFC6465 header extension is present. Currently, the
third_party/webrtc implementation does *not* fill in the audio level in
the CSRC case, but this CL takes care of the plumbing for both SSRCs and
CSRCs based on whether or not audio_level() is present in the webrtc
layer.

The getSynchronizationSources() test now passes.

Demo page: https://codepen.io/anon/pen/xmJVoy?editors=1010

Bug:  893172 
Change-Id: I9c3fac818c271875e6b14cf062057df034a5fc71
Reviewed-on: https://chromium-review.googlesource.com/c/1401521
Commit-Queue: Henrik Boström <hbos@chromium.org>
Reviewed-by: Harald Alvestrand <hta@chromium.org>
Cr-Commit-Position: refs/heads/master@{#621963}
[modify] https://crrev.com/86c8a3c5f1fee11f65b22897ef822cda7750ce23/content/renderer/media/webrtc/rtc_rtp_source.cc
[modify] https://crrev.com/86c8a3c5f1fee11f65b22897ef822cda7750ce23/content/renderer/media/webrtc/rtc_rtp_source.h
[modify] https://crrev.com/86c8a3c5f1fee11f65b22897ef822cda7750ce23/third_party/blink/public/platform/web_rtc_rtp_source.h
[modify] https://crrev.com/86c8a3c5f1fee11f65b22897ef822cda7750ce23/third_party/blink/renderer/modules/peerconnection/rtc_rtp_contributing_source.idl
[modify] https://crrev.com/86c8a3c5f1fee11f65b22897ef822cda7750ce23/third_party/blink/renderer/modules/peerconnection/rtc_rtp_receiver.cc
[modify] https://crrev.com/86c8a3c5f1fee11f65b22897ef822cda7750ce23/third_party/blink/web_tests/external/wpt/webrtc/RTCRtpReceiver-getSynchronizationSources.https-expected.txt