Imagery is description that uses the five senses of sight, sound, taste, smell, and touch. An aural image is a sound image.
In his poem "In the Sea," Reeves compares the sea to a dog. The great gray dog of the sea is sometimes hungry, sometimes restless, and sometimes calm and at peace.
Reeves uses sound imagery to help describe all three states of the doglike sea. When it is hungry, it makes sounds such as "clashing" its teeth, which would be a way to describe the crashing of waves. The sea/dog also "moans."
In the second stanza, the sea at night is compared to a restless dog. The winds that "roar" over the sea at night are likened to a dog that "howls."
Finally, the "reedy" sound of the calm sea is compared to the sound of a dog at rest, "quiet, so quiet," hardly snoring.
Whether these sound images work successfully is up to the reader to decide, but it is interesting to think of the sea as like a dog—frisky, howling, and quiet by turns.