The message contains a link: https://alfredvalley.itch.io/brute-fort-bonus
However it requires a password. I've looked through brief spectrogram analysis of all of the tracks and haven't found anything but i might need to look harder in dungeon drone. Other than that i haven't seen anything in the manual or PNP sheets.
(edit: upon further inspection nothing else seems to be hiding in the soundtrack itself, password must be elsewhere)