code: protected boolean checkSendMessageEnabled(RTMPMessage message) {
          IRTMPEvent body = message.getBody();
          if (!receiveAudio && body instanceof AudioData) {
              // The user doesn't want to get audio packets
              ((IStreamData<?>) body).getData().free();
              if (sendBlankAudio) {
                  // Send reset audio packet
                  sendBlankAudio = false;
                  body = new AudioData();
                  // We need a zero timestamp
                  if (lastMessageTs >= 0) {
                      body.setTimestamp(lastMessageTs - timestampOffset);
                  } else {
                      body.setTimestamp(-timestampOffset);
                  }
                  message = RTMPMessage.build(body);
              } else {
                  return false;
              }
          } else if (!receiveVideo && body instanceof VideoData) {
              // The user doesn't want to get video packets
              ((IStreamData<?>) body).getData().free();
              return false;
          }
          return true;
      }
docs: Check if sending the given message was enabled by the client.
  
  @param message
  the message to check
  @return true if the message should be sent, false otherwise (and the message is discarded)