Sentence structure plays an important role in the poem to establish the contrast between the first stanza and the lines that follow.
The inversion of the relationship of the sound to the object from the previous stanza ends here in the line
the door/scraped shut.
This change places the door in the more prominent position while the sound of it closing coming at the end helps to reinforce the assertion in the remainder of that stanza
it was the end/of all the sounds there are.
From here on the absence of sound in the poem is hugely important. The silence that allowed him to hear so keenly the sounds of the natural world has returned, yet this silence is oppressive and suffocates the speaker’s aural sense.
The shutting door is a metaphor through which MacCaig compares the door closing to the end of a relationship.
Just as a door closing creates a barrier between two places, so too the final moment of a relationship (the parting) represents the crossing point between togetherness and separation.
The speaker’s mood of despair is apparent - though we know a door can be reopened, here the speaker seems convinced that, once closed, this one will remain shut. There is no suggestion of the hope of a reunion.
The hyperbole of
it was the end/of all the sounds there are underlines the significance of the moment.
While before the speaker was delighted in describing the sounds of nature, now he shows us this pleasure has vanished. We get a clear sense of the painful despair that accompanies parting - the feeling that nothing will ever be the same again.