code: function forceStereoAudio(sdp) {
              var sdpLines = sdp.split('\r\n');
              var fmtpLineIndex = null;
              for (var i = 0; i < sdpLines.length; i++) {
                  if (sdpLines[i].search('opus/48000') !== -1) {
                      var opusPayload = extractSdp(sdpLines[i], /:(\d+) opus\/48000/i);
                      break;
                  }
              }
              for (var i = 0; i < sdpLines.length; i++) {
                  if (sdpLines[i].search('a=fmtp') !== -1) {
                      var payload = extractSdp(sdpLines[i], /a=fmtp:(\d+)/);
                      if (payload === opusPayload) {
                          fmtpLineIndex = i;
                          break;
                      }
                  }
              }
              if (fmtpLineIndex === null) return sdp;
              sdpLines[fmtpLineIndex] = sdpLines[fmtpLineIndex].concat('; stereo=1; sprop-stereo=1');
              sdp = sdpLines.join('\r\n');
              return sdp;
          }
docs: forceStereoAudio => via webrtcexample.com requires getUserMedia => echoCancellation:false