Can you tell us a bit more about your scenario?
NoMachine Microphone feature is based on inputs’ audio devices. On the Client side, as an input, you should have set a physical device, which from what you have written I assume you have done. On the Server side it should be NoMachine Microphone Adapter.
Forwarding the mic from the client to the server allows the user to use a remote side app (like Skype for example or some app in which you want to record your voice). Another important thing, there is no loop back from the microphone with NoMachine. That means you won’t be able to hear the microphone directly, but the application using the microphone will. For example: using Skype in a call to someone from the remote computer. Your colleague or friend will hear your voice on their computer, your voice will not be heard on the computer where your Skype session is running.
You can make a quick test during NoMachine connection by pointing your remote browser to this website after you’ve forwarded the client-side mic to the server.
https://www.onlinemictest.com/
Your browser will detect mic, despite you not being able to hear it.