This article lists some of the most common cases of audio/microphone issues occurring when running NoMachine physical desktops or virtual desktops/custom sessions on RHEL-based systems like CentOS 7.
 

Audio is missing in the NoMachine session

1) Download PulseAudio Volume Control

2) Launch PulseAudio Volume Control,

3) Go to “Output Devices”

4) Click on the button “Set as fallback” (green button) next to the Built-in Audio Analog Stereo

5) Increase the sound level.

Audio is missing, NoMachine audio devices are not available

If there is still no sound after increasing sound level in PulseAudio Volume Control, you can check if NoMachine audio devices are available on the system.

To do that, execute in a terminal:

pactl list sources | grep NoMachine
pactl list sources | grep nx
 

If the list is empty, you can recreate NoMachine sinks and sources in the following way:

1) Kill the PulseAudio process, which automatically spawns a new process.

    Execute in a terminal:

  pulseaudio -k

2) Restart the NoMachine server. 
       
    Execute in a terminal:

  sudo /etc/NX/nxserver --restart

Microphone is not working

Please see: https://www.nomachine.com/AR11P01006