Why do you think the auditory images (“Listen!”) are given such importance?
In answering this question, one should remember that the speaker is addressing his wife at nighttime ("The sea is calm tonight"). Although the moon is shining, the lights on the French coast have gone out, casting a pall of darkness upon the beach. The loss of light inevitably means that the speaker puts more stress on sounds than images.
And what sounds! The loud, "grating roar" of the waves drawing back over the pebbles...
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