Does KDE even need a sound server?

Not really, though plasma-pa is better integrated than kmix and IIRC the kde-standard metapackage pulls plasma-pa or kmix, with preference to the former.
ALSA not working is likely one of the usual suspects (e.g. muted master PCM or wrong default card), but OP was futzing with pipewire and close to a reasonable solution, so that's what I rolled with. (also an excuse to taunt gotard OFC, no reason to miss one of those)

I'm unable to find any installation of Devuan Excalibur with KDE Plasma DE on which sound works out of the box.

That's because it (still! ) doesn't.

See the release notes and the relevant thread argument.

May or may not work, subject to the phase of the moon (service startup is order dependent and 'foo &' doesn't wait). See better solutions further down the aforementioned argument.

Devuan's stubborn refusal to implement any of the multiple user-service solutions already suggested, or package even a token-effort shell script.

As the topic says, I'm unable to find any installation of Devuan Excalibur with KDE Plasma DE on which sound works out of the box.

I don't have the needed knowledge to know what produces this, and I'm tired of investigating about that, so any help is welcome

Troubleshoot I've done:

-> Sound works out of the box when installing Devuan Excalibur with XFCE DE

-> Sound DON'T work installing Devuan with Plasma DE with any init, including SysVinit, OpenRC or runit.
There are installed some Pipewire, alsa and pulseaudio packages, so I don't think that don't having X package is the problem.

On Plasma, I've tried this by reading some similar new topics on this forum, that allows to sound to work. On a fresh-installed Devuan Excalibur Plasma system:

-> Create a .sh file, for example pipewire.sh
-> Write on that file the next:

#!/bin/bash
wireplumber &
pipewire &
pipewire-pulse &

-> Make that file executable
-> Run terminal on pipewire.sh location folder, then inside it, run ./pipewire

Sound should work right now. To make that works everytime session is booted: System Settings -> Autostart -> Add New, then select the .sh file you created before.

So, I like to know why something work or don't work on my system, and I am very surprised that this thing (dont having sound work out of the box) dont affect to some people, except someone here for example (https://dev1galaxy.org/viewtopic.php?id=7776).

Help is very apreciated, as I have looked for a solution for weeks and I'm tired up. I'm using Devuan as I can't hibernate my laptop with Systemd, and I liked a lot using Devuan with XFCE, but I need to use a Qt based DE because I almost only use Qt apps, and personally I like more Plasma... please, I would like to know why this problem happens to me and where does it comes from, I like to know why things happen!

UPDATE: if I log off, sound will not work unless I run manually ./pipeware.sh on a terminal. Pleasee help