If sound is played in NoMachine 4 sessions using RealPlayer the following error may occur:

Opening ALSA PCM device cards.pcm.default
ALSA lib confmisc.c:768:(parse_card) cannot find card '0'

Solution:

in the RealPlayer menu under tools -> preferences -> Hardware Audio Driver,
set the Alsa PCM Device to 'pcm.default' or 'default'.

If then a library error occurs, be sure that the plugin for PulseAudio is installed, usually along with this package: alsa-plugins-pulseaudio.

Note that RealPlayer is a 32 bit application and therefore needs 32 bit libraries.
Some operating systems like RHEL6 install only 64 bit libraries by default.