Blow, bugle, blow, set the wild echoes flying (5)
When the speaker first hears the bugles, he doesn't say, "Oh, huh—there must be someone else out here today practicing the bugle." Instead, he addresses the bugle directly, as though he wants to pretend that there's not really anyone else out there. Nope, just a bugle playing itself!
Blow, bugle; answer, echoes, dying, dying, dying. (6, 12)
The speaker repeats the same line twice, again addressing the bugle directly (this is called apostrophe—see the "Symbols" section for more on that).
O love, they die in yon rich sky (13)
For the first time, we're given a hint that the speaker might not be alone! He addresses someone as "O love." Of course, he might be addressing someone who is absent, and he might be addressing someone who has been at his elbow the whole time—it's hard to say. What do you think?