I just installed Arch x64 and KDE and I'm having some trouble with KMix + PulseAudio. I have sound, I can hear sound from Youtube videos, VLC, Spotify... but just each one of them at a time. I can't open VLC, start playing something and then open a Youtube video. Maybe this isn't KMix's fault but PulseAudio?
The subject of the topic is because KMix doesn't seem to detect more than one application at a time, so I can't control different apps's sound level.

How can I fix it?

maybe (not sure) that is a function of Phonon and for control of multiple sources you should install Veromix (KDE)of Pavucontrol (gtk?)

No, it's not because of pavucontrol or veromix. The problem is that once some application is using my sound card, no other application can access it. It's like the very old days with OSS when only one application could access the sound card.

Ok, this can be closed. It turns out that I forgot to install pulseaudio-alsa and lib32-alsa-plugins . Everything works properly now

I would also recommend installing the appropriate Pulseaudio plugins for the Phonon backend you are using - usually GStreamer or VLC. This will ensure minimum resource use and minimise issues.