Depending on the sound card on the Windows host, it might not be possible to mute audio on the NoMachine server side. If the Windows audio driver doesn't provide functionalities like 'mute control', the WASAPI loopback capture can intercept only silence, when the server is muted.

Unfortunately there is no way to workaround this behaviour, but it's possible to install and use the NoMachine Virtual Audio Adapter (NXVAC). On recent Windows versions like Windows 10 and 11, NoMachine installs only the Microphone Adapter, but it's still possible to install the Audio Adapter manually.

To do that, open a CMD console running as administrator and follow instructions below.

1) Install NoMachine Audio Adapter:
> cd C:\Program Files (x86)\NoMachine\bin
> ./nxservice64.exe --deviceadd --inffile drivers/nxaudio/i386/nxaudio.inf --hardwareid *NXVAC

2) Check if NoMachine Audio Adapter is properly installed:
> cd C:\Program File (x86)\NoMachine\bin
> ./nxservice64.exe --deviceslist --hardwareid *NXVAC

3) Finally, select the NoMachine Audio Adapter as your default audio output.

As an alternative, you may try to update your audio driver or try a different one.

Notes:

a) When the NoMachine Audio Adapter is configured as default audio output on the system, audio will be no longer played locally. To restore local audio, revert to the Windows audio adapter.

b) To uninstall NoMachine Audio Adapter:
> cd C:\Program File (x86)\NoMachine\bin
> ./nxservice64.exe --devicesgroupdel --hardwareid *NXVAC