[LVL ED] Do Kaernks (Water monsters) need pathnodes?

[Your Computer]

Don't they just react on the sound that occurs near a pathnode?

A pathnode is not required, in that the water lurker would still attempt to head towards the sound; but at the time the sound is heard, the game searches for the nearest pathnode and will use that if any as guidance.

But can they navigate a maze without pathnodes?

Depends how far behind it is from the player or whatever made the sound. Placing random path nodes can help the monster along.