When microphone forwarding is enabled, Ubuntu does not expose the client microphone as a real input device. In the system audio input settings, only “Remapped nx_voice_out”  is available and it is handled as an output stream, not as a microphone. As a result, audio is played through the speakers and forwarded back to the client, creating a feedback loop.

The client microphone should instead appear on the Linux server as a proper input source selectable by applications, without audio being routed to local outputs. 

Speaker audio forwarding from the Linux server to the Windows client works as expected.
 
This appears to be an interaction issue between NoMachine audio forwarding and PipeWire/PulseAudio on Ubuntu 24.04.

Due to the incorrect creation/mapping of the microphone, the remapped nx_voice_out is also duplicated.