Hi,
Do you want to transmit audio stream from the input device “PCM2901 Audio Codec Analog Stereo” to the client side (Ubuntu and MacOS)?
NoMachine uses default sink (output device) on the server side (Linux), then reads audio stream using monitor of this sink and transmits data to the other side. You cannot change this monitor when NoMachine start to record audio. That’s why you are not able to select source other than “Monitor of PCM2901 Audio Codec Analog Stereo” in the Recording tab of pavucontrol.
What is your default sink (output device) when the NoMachine session is running? You can check this in the pavucontrol or by using command in terminal:
pactl info | grep Default