KDE doesn't provide sound, just like KDE doesn't provide graphics drivers. I'm not sure why you feel sound support should be a function of the desktop environment.
My opinion on what should be a part of what is irrelevant.
My post was regarding the fact that people blame Linux when the real problem is their own lack of understanding.
A good answer to the original question would be: "Sound is not a problem in Linux. However, compared to other operating systems, it might require additional understanding of the way things work, especially if you want to customize your system. This requirement of knowledge is a problem in distributions like Ubuntu - where the user base is 'everyday people'."
Phonon is not a mixer. Phonon is essentially a built-in adapter layer over whatever audio driver and media decoder libraries you already have.
It's a little more complex nowadays as it was substantially modified to be a good PulseAudio citizen when running with PulseAudio, but Phonon doesn't talk to hardware, it talks to sound libraries such as VLC, gstreamer, or PulseAudio.
1. Remove your entire desktop environment,
2. Install just a window manager,
3. Now you don't have any program handling your sound, you just removed it, remember? (see (1) for reference),
4. Post on HN: "Yes, sound is still a problem."