Possibility that the event can only be triggered by certain conditions that can be met. For example, if the audio is embedded in a geolocating playback format, the walk cannot be passively listened to at home.
This variable is the possibility of insisting that the walk is an event. Like a theater show that can only be experienced in a particular place at a particular time. Audio Walks might dispense with the exact time, although a walk that could be tuned in from a low-power fm pop-up radio station in a particular area could be a way to re-impose start times on the walk.
Using limitations this way makes it harder to experience the thing, but perhaps more special when it is experienced.