I'm a doctor, not a mechanic
This is a quick way to add position based ambience (amb. that can move) to your map. Generally you have a radius in UU, a fall off radius, overal volume, and of course occlusion type. (See ESoundOcclusion on Actor/Enums for a description of the occlusion types.)
This actor was coded for Ravenshield. It is not present in UT2003 or UT2004.
Foxpaw: Any actor with an ambient sound can be moved around.. what's special about this actor?
Mr. Lane: This description is completely wrong. Dynamic ambient sounds are used to create one-shot randomized sound effects such as bird calls/thunder/machine sounds. You can assign up to 16 different sounds and specify the maxiumum and minimum time between sounds being played. The engine randomly chooses a time between these values and plays one of the sounds at random. From memory you can also tell it not to play the same sounds twice in a row. Unfortunately you cannot alter the radius or volume of the sound. Plenty of these can be found in Unreal 1, especially in the more sound rich early levels, but they use that standard keypoint icon. These have been superceeded in Unreal Engine 2 by additional functionality added to the AmbientSound Actor.
MythOpus: Though they were present in Unreal, they could have been recoded to behave differently in 'Ravenshield'.
SuperApe: Am I missing something? Is this a Custom Class page? If not, what is it? Where is it linked from? What's it's purpose?