How I found it (in case anyone cares):
At the end of track 6 (Dungeon Drone), there is a man speaking backwards. When reversed, he says https://alfredvalley.itch.io/brute-fort-bonus. The link takes you to a password-protected download page. Track 5 (Chains of the Divine) features the only other spoken word on the soundtrack (forswear). I plugged that in as the password and lo! track 7 is unlocked!