In ‘sings’ and ‘strain’, “s” sound has been repeated. The use of the same letter or sound at the beginning of words that are close together is quite common in poetry. This “poetic repetition is called “alliteration”. It is used to heighten the musical and lyrical impact of the poem.
Repetition of “s” sound in ‘silence of the seas’.
Repetition of “a” sound in ‘Among Arabian Sands’.
Repetition of “1” sound in ‘Listened, motionless and still’.