When connected by NoMachine to a KDE desktop v. 4, the microphone may not work properly inside the NoMachine session. KDE 4 multimedia support relies on Phonon subsystem.

Although the NoMachine Microphone is set to be the default device, programs may ignore this. They therefore record audio input from other devices. As a result, the audio signal from NoMachine Microphone differs from the actual stream or is even muted. 

As a temporary solution, configure the Phonon framework for NoMachine:

Go to Audio Setup -> Audio Recording and change the order of the list in order to have NoMachine Microphone at the first position.

If this doesn't help, download and install the PulseAudio Volume Control (pavucontrol) as explained below. Execute the following instructions from a console.
 

On RHEL-based systems

1) Import NUX Dextop Repository:

rpm --import http://li.nux.ro/download/nux/RPM-GPG-KEY-nux.ro

2) Install NUX Dextop Repository and Fedora EPEL

    for Centos/RHEL 7

yum -y install epel-release && rpm -Uvh http://li.nux.ro/download/nux/dextop/el7/x86_64/nux-dextop-release-0-5.e...

    for Centos/RHEL 6

yum -y install epel-release && rpm -Uvh http://li.nux.ro/download/nux/dextop/el6/x86_64/nux-dextop-release-0-2.e...

3) Install pavucontrol

yum install pavucontrol

On DEB-based systems

sudo apt-get update

sudo apt-get install pavucontrol

Once pavucontrol is installed, check the recording devices in the PulseAudio Volume Control panel.

Recording devices for different applications (except NoMachine) should be set to NoMachine Microphone.

If they are not, execute in a terminal:

pactl unload-module module-stream-restore
pactl load-module module-stream-restore restore_device=false

As an alternative for experienced users, it's possible to edit the PulseAudio configuration file, namely:

/etc/pulse/default.pa

and add the following entry for the module-stream-restore:

load-module module-stream-restore restore_device=false