The River-Merchant's Wife: A Letter
In terms of the strategy at work in crafting the sound of this poem, it's easier to talk about tone than it is to talk about specific techniques. If you really twisted our arm, we could point out that there seems to be a lot of consonance at work here, and it seems to be stanza specific. For example, there are a lot of S sounds in the first stanza, followed by L sounds in the second, D's and F's in the third, S's and M's in the fourth, and then back to more S's in the fifth.
So, what's that mean? Well, possibly nothing at all. If we go back and look at the poem as a whole, we do notice a dominance of the S sound in general, which might be intentional on the part of Pound in order to suggest the shape of the river described here, the sound of the water, or the sighs of the wind. At any rate, the use of sound—if we do want to claim that it's intentional— is really in the service of the poem's emotional impact on the reader, which is why we come back to tone.
As a letter, this poem presumes familiarity (from a wife to her husband). So a casual tone predominates, with the childhood reminiscing taking place in long, unhurried lines that might suggest the way a couple would converse about random stuff, with no urgency to their conversation. Too, that casual, languid tone sets us up as readers for a sharp turn at the end of the poem, as it plunges right into the power-packing lines of 25 and 26. Those lines stand out because a) they are the most direct in terms of the speaker's emotional reporting, b) they represent a break from the longer lines that have come before, and c) when read out loud, they sound like they deserve special consideration from the reader for being so radically different.
To put it in the poem's terms, the sound of this poem is like floating on a slow, lazy river, then hitting some unexpected rapids. The impact is immediate, and the emotion is visceral. The poem's entire focus is to render the emotional state of its speaker, and that's something that comes across even in its sonic design. Pound, you magnificent maniac, you've done it again!