Please confirm that I understand what you want to achieve: you are connecting to your friend’s computer whilst your friend is sitting in front of his computer. You want to talk to each other inside the NoMachine session, like VoIP software ? If that is the case, NoMachine does not support two-way microphone, i.e it is not a “voice chat” application.
What can NoMachine let you do? NoMachine lets you connect to a remote desktop to view and interact with what’s showing on that desktop, and work with applications installed that computer, which might also be VoIP applications.
For example, you could start a VoIP application installed on the remote desktop and use it to call a friend who is somewhere else. Enabling microphone allows you to forward the functionality of your local mic to the remote desktop so that you can use VoIP applications residing there.
By default microphone is always disabled. Users have to activate it from the NoMachine menu panel inside the session. Click on the right upper corner of the session window or press Ctrl+Alt+0 to open the menu.
Choose ‘Mic in’ icon button and click ‘Enable mic’. Sections 6.1 and 7 (Fig 18 and 19) in the following guide:
https://kb.nomachine.com/DT06S00218#6.1