If you do not hear sound (ambient noise) from the cameras when recording the CMS-30300 system, this may be due to the following reasons.  

Make sure that the connected HDMI monitor has speakers. For example, there are also PC monitors that do not have built-in speakers. If the HDMI monitor has speakers, check the volume settings.

If there is still no sound in the recording, check the following setting

 Recorder menu---system setup---channel setup,  you choose “AV stream” , then click “Apply”

Now sound should be recorded and played back with your recordings.