لَا يُسْمَعُ إِلَّا وَقْعُ خُطَاهُ.
Nothing was heard except the sound of his footsteps.
Negation and exception restrict what can be heard to the traveller’s footsteps. The domain is the immediate narrative scene, not every sound anywhere in the world.
The restriction becomes vivid after the statement about the silent road: a single sound emerges against an established quiet background. Its force is partly created by sequence.