There is an issue with the way KDE installs in Arch Linux. This fix I discovered on the internet may help you.
If KDE does not recognise your sound device, but it is clearly being discovered by ALSA and Pulseaudio, it is because the Pulseaudio daemon is not being started at the right time before Plasma loads.
The fix to start the daemon at the right time is very simple. It is this:
nano -w /etc/pulse/client.conf
autospawn = yes
You can find the original explanation of this fix at this URL: https://forum.manjaro.org/t/kde-no-devices-in-audio-volume-settings/11849
If the above fix produces a pulseaudio which cannot detect your soundcard, and KDE is only showing a Dummy Output, this is the fix:
pacman -Rsnc fluidsynth
There is a conflict between pulseaudio and the above package for some reason. The explanation can be found here: https://bbs.archlinux.org/viewtopic.php?id=154002