In “The Splendor Falls,” the first stanza paints a picture of a sunset. We know this because of the phrase “long light.” This means that the sunlight has been shining for a long time (all day) or that, being close to the horizon, the sunlight is long, not directly overhead, and is casting long shadows. The cataract, a waterfall of some kind, flows with life. The splendor of this beautiful scene does fall. The water falls and the light falls upon the earth. But this is not a completely melancholy mourning of the end of the day. So, it is not completely an analogy between the day ending and, say, the ending of life. The speaker calls for bugles to play as a celebration of the beauty of nature. That the echoes from the bugle eventually fade (“dying, dying, dying”) is a tribute to the ending day.
In the second stanza, the speaker characterizes the replying echoes as if they are answers from “Elfland.” The comparison is to give the echoes a magical or fairy-tale quality. In fact, bestowing the echoes with the quality of coming from another place and another time gives the echoes a timeless quality. That is, even if they fade, they seem to communicate beyond time.
In the third stanza, the speaker addresses his lover.
O love they die in yon rich sky,
They faint on hill or field, or river:
Our echoes roll from soul to soul,
And grow forever and forever.
The speaker comments on the echoes of the bugles and then he speaks of their love; “Our echoes,” roll, not through the air, but from “soul to soul.” Their love does not fade like the audible echoes. This is because, like the timeless quality of Elfland--communicating between two worlds, their love is preserved over time, beyond history. Perhaps this is because their love is preserved in writing, in this poem, or it is because echoes of love between souls, being metaphysical, are not subjected to the limitations of the physical world.
Notice the last line in each of the three stanzas. In the first two stanzas, the last line is, "Blow, bugle; answer, echoes dying, dying, dying." The bugle plays; and the echoes of that sound answer the bugle. This is physically how echoes work. In the third stanza, the echoes are metaphors for the love between the speaker and his lover. The line reads, "And answer, echoes, answer, dying, dying, dying." Here the echoes are answering themselves, echoing (pun intending) the reciprocating nature of love. An echo that can answer itself will last infinitely longer than one that just fades or "falls" away.