The rules seem to imply that the undead have to be within range and sight/earshot only at the beginning of the effect. Which makes sense since part of the feature is that they run away from you and would then cease to be within range at some point. However, what happens if the cleric becomes unconscious or even dies, or is teleported to another city/continent/dimension? The undead would no longer know where to run to since they don't know your location. This leads to an interesting set of questions:
Do the undead track the cleric's location while they are running or are they just too afraid to care to look back? If this is the case, what happens if the cleric moves sideways and the undead running away from the cleric's original position would bring them closer to the cleric?
The implication of the second paragraph seems to be that they will perceive the cleric's new position only if they are getting within 30 feet of them. But then what if the cleric teleports or otherwise moves directly in front of them? Will they just keep running blindly until they reach the effect range and then turn heel?
I suppose the ultimate question is if the cleric is the source of the "fear effect". This seems to be implied by the rule saying that the undead try to run away from the cleric. Which would mean in turn that a disabled or relocated cleric would no longer pose a threat to them if the undead are aware of the cleric's new condition.
PHB page 59: Channel Divinity: Turn Undead
As an action, you present your holy symbol and speak a prayer censuring the undead. Each undead that can see or hear you within 30 feet of you must make a Wisdom saving throw. If the creature fails its saving throw, it is turned for 1 minute or until it takes any damage.
A turned creature must spend its turns trying to move as far away from you as it can, and it can't willingly move to a space within 30 feet of you. It also can't take reactions. For its action, it can use only the Dash action or try to escape from an effect that prevents it from moving. If there's nowhere to move, the creature can use the Dodge action.