So this evening when I got back from the Lurps meal I was looking around online and stumbled across Boodler, which is a program designed to create sound scapes, i.e. if you give it a library of sound effects you can configure it for how often you want each to play and within what kind of ranges and it layers them all over each other. Written in Python, source available, downloadable packages for Windows machines.
Fiiiine :) I pondered writing something like this a while back for use as a soundtrack generator for games, but nice to see someones done all the heavy lifting.
Be see to read the fine manual hard to make it work without that.