It happens from time to time, and can be solved rebooting or restarting the session. However it is really annoying making a reboot once you have all your toolbox of programs running.
The solution is as simple as opening a Terminal (ctrl + alt + T)
and restating the pulse audio daemon:
pulseaudio -k (no need to be root)
pulseaudio --start
pulseaudio --check
now you can use you media player with sound!
