A heatmap is a type of map that is used not just in the game, but when viewing data. It is used to depict how much of something is in a given area. A population map is often shown as a heatmap because it easily shows how densely populated areas are in a way that's easy to understand. The game makes use of a similar thing, where the more activity you do within an area, the higher the heatmap value gets. It really has nothing to do with heat, though some things like a campfire or forge do increase that value. But those increase it because of the activity and not because of the actual heat. As far as what the activities are that generate "heat," that is up to what they set up in the XML. Actual sound doesn't increase it, but things that make sound can be set to generate the heat. So mining will increase the heat, but it's because of the activity and not specifically because of the sound. So if you just modded out the sound, the heat would still increase. You'd have to instead mod out the heat increase value.