Fix stereo playback in Chrome

See
  https://chromium.googlesource.com/external/webrtc/+/194e3bcc53ffa3e98045934377726cb25d7579d2/webrtc/media/engine/webrtcvoiceengine.cc#302
  https://bugs.chromium.org/p/webrtc/issues/detail?id=8133

Technically it's against the spec to modify the SDP but there's no other
API for this and this seems to be the only possible workaround at this
time.