Cousin_luigi said in #kde tonight: "I expect the volume to be at the same level I left it." Which is perfectly reasonable! And yet, we often find something else. Axtroz had the answer:
Cousin_luigi, because some systems run "alsactl restore" on startup which restores the volume state saved with "alsactl store" and that tunes the volume for a particular soundcard. Since Pulseaudio and Alsa are working together via plugins, pulse follows. Check your init scripts, or raise the volume to an apropriate level and run alsactl store as root.
I don't have this problem, so I didn't test the solution. However, here it is as a public service. Thanks to Cousin_luigi for asking, and Axtroz for answering.