This is an example of sounds implemented using an audio middleware application called ISACT (Interactive Spatial Audio Composition Technology) from Creative Labs
You can see the original post from 2006 at the 0 A.D. Development Blog @ blog.lostchocolatelab.com
There are 2 "Sound_Events":
These are loaded up with 4 agricultural ambients (same in either sound event).
These Sound_Events playback the 4 files in various ways:
1. In a user specified order.
2. By chance, with various ways to determine the potential of them playing
(with provisions for preventing repeats)
There is then a "Sound_Entity":
Created in order to enable multiple players/channels/streams to play simultaneously based on definable action variables like state changes, etc.
The sound entity is being used in this example to control when either 1 set of Sound_Events are being played (based on the parameters for randomization), or both sets play simultaneously (based on the Sound_Entity action definitions).
For more information about ISACT and other Game Audio tools please check out the Audio Implementation Greats Series over at DesigningSound.org