Default settings ?
At here, Kubuntu 13.10 - ffmpeg 2.1.4
The default, at here, is the PulseAudion Control: 'Built-in Audio Analog Stereo' and the capture doesn't have the sound.
I need to change the Alsa Capture to the Monitor of Built-in Audio Analog Stereo. Then the capture has the sound.

The drill at here:
a) Start the pavucontrol
The PulseAudio Volume Control or pavucontrol is:
:~$ apt-cache show pavucontrol
Package: pavucontrol
Priority: optional
Section: universe/sound
...
Description: PulseAudio Volume Control
PulseAudio, previously known as Polypaudio, is a sound server for POSIX and
WIN32 systems. It is a drop in replacement for the ESD sound server with
much better latency, mixing/re-sampling quality and overall architecture.
.
PulseAudio Volume Control (pavucontrol) is a simple GTK+ based volume
control tool (mixer) for the PulseAudio sound server. In contrast to
classic mixer tools this one allows you to control both the volume of
hardware devices and of each playback stream separately. It also allows
you to redirect a playback stream to another output device without
interrupting playback.
Homepage: http://0pointer.de/lennart/projects/pavucontrol/
b) Start the ffmpeg script
c) Pick a sound source
At here the pavucontrol has options:

ffmpeg
console output.