Comment 4 for bug 447844

The issue at hand is actually that, unless you've manually adjusted
Phonon's preferences in System Settings, you've got two settings that
clash if you have PulseAudio installed(, too):

1) by default, Phonon ships, in Kubuntu, with ALSA's 'default' virtual
device as the primary;
2) if you somehow have acquired PulseAudio, it starts automatically
("autospawns") whenever an ALSA client attempts to access the
'default' virtual device.

Now, you would think that, given that clients accessing 'default' are
routed through PA, everything is peachy. But this is (obviously) not
the case. Apps can still use the 'plughw' virtual device or the 'hw'
virtual device (the latter of which PulseAudio does, to a degree),
both of which will override/take preference over 'default', which
means that once PA resumes from its two-second idle period (during
which it relinquishes the audio devices), it will find the audio
devices in use and fall back to the dummy/null sink.

This explanation is a very long way of saying the following:

If you use Kubuntu and have PulseAudio installed, you need to
configure Phonon in System Settings to prefer the PulseAudio
output/input for all the profiles, otherwise you're flirting with
disaster! (just joking about disaster, but, well...)