I finally managed to understand how to change dynamically the system beep with pulseaudio. I do not claim it was complex, but it was not instantaneous.

The default sample is referenced (in Debian) in /etc/pulse/default.pa (near line 87) with a line looking like load-sample x11-bell /usr/share/sounds/gtk-events/activate.wav. The default volume is the one set in the X11 properties. Alas, it is not changeable at all in the Gnome preferences, but a simple xset b 100 is sufficient in any startup program (System -> Preferences -> Sessions). If the default level of 50% is enough for you, do not change it.

The first thing is to suppress the ordinary beep: as root, add at the end of /etc/modprobe.d/myblacklist (or create the file with) the line blacklist pcspkr. Proceed then to the removal of the currently running pcspkr kernel module with rmmod pcspkr (do all of this only once).

Being root and editing /etc/pulse/default.pa is not convenient, but pactl gives any user the possibility to redefine its own beep (and change it dynamically, thence):

#!/bin/sh
# file saved as ~/bin/setbeep and set as a startup program
xset b 100
SOUND="${1:-/data/macossounds/Quack.wav}"
if [ ! -f "${SOUND}" ]; then SOUND="/data/macossounds/${SOUND}.wav"; fi
if [ -f "${SOUND}" ]; then pactl upload-sample "${SOUND}" x11-bell; fi

I can now use again the old beeps I salvaged once on an old Macintosh, and listen again to my old-time favourites such as "Quack" or "Boing"... or even play a whole three-minutes score each time I hit a wrong key!

PS: For obvious copyright reasons, I cannot attach those fabulous beeps, but I do not doubt that any agile mind can find those somewhere on the net.