Until now I have never heard a sound played by the esound
daemon of Gnome in Leopard.

I have just discovered that if you export "ESPEAKER=localhost"
in the shell from where you launch a Gnome app
(i.e. gnome-sound-properties), the sounds are played again.

This tip could be mentioned in the documentation of the scripts
to launch Gnome for the benefits of those who are still interested
in testing (and hopefully maintaining) it.