Linux, Sound and blacklist.conf

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