Which poetic or sound device does robert frost use in the line "the only other sound's the sweep," from his poem "stopping by woods on a snowy evening"?

Consonance and assonance

This line uses consonance with the repetition of the 's' sound at the beginning and ends of words. The 's' sound is repeated three times:
"The only other Sound'S the Sweep,"

The line also uses assonance, with the repetition of the 'o' vowel sound. The 'o' sound is repeated three times as well:
"The Only Other sOund's the sweep,"