This poem is full of alliteration, which helps to intensify its imagery. Sometimes, this creates a peaceful, idyllic atmosphere that is sadly too good to be true; in other moments, it does the complete opposite, conjuring up fright and horror.
In the first two stanzas, alliteration helps lull the reader into a false sense of security. The dying man drifts in and out of consciousness:
Soaring and quivering in the wings of sleep.
Silence and safety; [...]
These sibilant sounds feel gentle and whispery, almost like sleepy breathing.
And the following stanza uses dense alliteration to even more musical effect:
Water—calm, sliding green above the weir;
Water—a sky-lit alley for his boat,
Bird-voiced, and bordered with reflected flowers
And shaken hues of summer: drifting down,
He dipped contented oars, and sighed, and slept.
Put simply, all of these little echoing sounds make for an extremely pretty passage. If the poem ended here, the soldier's death wouldn't seem so bad at all!
But the whole point of this section is that it is too pretty—that is, it's not real. The comfort and security of the first two stanzas ultimately exist only to make the ending more chilling.
Contrast the above passages with lines 29 and 30, which describe the young man's sudden pain just before he dies:
He stirred, shifting his body; then the pain
Leaped like a prowling beast, and gripped and tore
His groping dreams with grinding claws and fangs.
These grating /gr/ sounds and slap-like /p/ sounds suggest violence, and all the more so because of their contrast with the sweet sounds earlier in the poem.
At the end, the poem returns to sibilant /s/ alliteration:
But death replied: “I choose him.” So he went,
And there was silence in the summer night;
Silence and safety; and the veils of sleep.
But now these /s/ sounds don't suggest comfort, but the creepy whisper of death.