The sound tracing is a system whereby I trace sounds from their source, through portals, towards the listener ( usually the player, but sometimes the camera ).
When the listener is in a different cell than the source, then the sound appears to come from the portal, rather than the actual sound source. The volume is also attenuated based on the linear distance from the source, to portal -> portal -> listener.
There were latent bugs in the sound tracing system that were only exposed when we expanded the 1st level of the game - issues where there were multiple paths from the source to the listener, each one of similar distance. There were some overly aggressive optimizations not to re-consider a cell already considered which were causing shorter sound paths to sometimes not be considered. This caused a sound to be heard when touching the portal, but not heard ( due to being culled ) when just outside the portal. Very distracting.
For a while I experimented with disabling the system, and just leaving the sounds at their true source positions, but since the sound tracing feature has been around so long, the levels sounded really bad without it.
I ended up making several fixes, each of which would solve another issue, which would expose another bug, some of which were recently introduced. Anyways, right now I believe the sound tracing system is close to bug-free.
Another small change I made was to make closed doors muffle sounds, in proportion to how closed they are. A nice effect when you are opening a door into an area.
There are many, many gotchas when doing a sound tracing system. Perhaps I will write them up one day.
The other change was pretty minor in terms of code, but seems to add proportionally more to the game is the ability of NPCs to use power-up crystals if they have one in inventory. They don't always use them, however, so the player can find some on NPC corpses.
At this point, we're just making some minor adjustments and fixing any remaining issues that come up, and then we will release the next version.