The Wet Mix value determines the amplitude of the filtered signal, where the Dry Mix determines the amplitude of the unfiltered sound output.
Hard surfaces reflects the propagation of sound. For example a large canyon can be made more convincing with the Audio Echo Filter.
Sound propagates slower than light - we all know that from lightning and thunder. To simulate this, add an Audio Echo Filter to an event sound, set the Wet Mix to 0.0 and modulate the Delay to a RTP that is controlled from the built in Distance parameter.