Can you mention the sound file(s) that should play? Might be that the minimal install lacks the needed sound files. Or have you tried associating a different notification sound file with the events?
Many sound files are in the oxygen package, show the output of
pacman -Qs oxygen (even when empty, mention that).
Scratch the above, you found a bug, knotification is build missing the libcanberra/sound-theme-freedesktop packages, thus no notifications (oxygen shouldn't be needed anyway). Fixed version will be up shortly.