Did you manage to find a solution for this, we're having a similar issue where the event only plays on the root object. Here's the code, would be great if someone could look at it.
//River to array
AkPositionArray arr = new AkPositionArray((uint)river.transform.childCount);
foreach( Transform child in river.transform) {
Vector3 pos = child.position;
Vector3 forw = child.forward;
Vector3 up = child.up;
arr.Add(pos, forw, up);
}
AkSoundEngine.SetMultiplePositions(river,arr, (ushort)river.transform.childCount, MultiPositionType.MultiPositionType_MultiSources);
AkSoundEngine.PostEvent("ambient_river", river);