The (incomplete) Thief2 and System Shock2 source code was leaked years ago from a Dreamcast development kit. I imagine a Google search would find you a download. I think I saved a copy too, someplace.
A lack of time and energy has a way of killing interest and desire. I expect if you managed to attain or retain more of both of those you’d find some level of passion. And passion doesn’t have to be earth shattering. There’s a lot of joy to be found in the small stuff.
Setting up the audio propagation in the Thief level editor is super tedious but it’s hard to argue with the results. You draw “Room brushes” (basically boxes) to encompass space and sound will propagate across where they intersect with each other. An Environmental audio setting is applied to a room brush and will be applied to all the audio heard by the player when inside the room brush.