Identifying Assonance in Poetry

Read the following excerpt from a poem:

"In the stillness of the night, a shadow creeps,
Over the hills where the ancient wisdom sleeps.
The owl calls softly, a haunting refrain,
As whispers of dreams dance through the rain.
The echo of silence carries the weight,
Of secrets unspoken; they patiently wait."

What sound device is primarily employed in the lines, especially emphasizing the auditory effect?