For some reason, the KMix configuration window pops up on login for me sometimes. I don't know why it starts, but there's not an obvious way to stop it.
After some Googling, I found a forum post with a fix: https://www.linuxquestions.org/questions/linux-software-2/kmix-window-on-startup-278600/
Save then Logout/Login and it shouldn't pop back up again.
Alternatively, you could just remove ~/.kde/share/config/kmixrc
Make a backup first.