Forget about configuring your sound device with KDE.
It is just confusing and does not work as you intended most of the time.
Here two useful blacklist.conf entries:
Disable the onboard soundcard in favor of USB soundcard:
blacklist snd_hda_intel
Disable the HDMI output of your soundcard:
snd_hda_codec_hdmi
pavucontrol is a useful tool to mute speakers and adjust the balance