The breezy call of incense-breathing Morn, The swallow twitt'ring from the straw-built shed, The cock's shrill clarion, or the echoing horn, No more shall rouse them from their lowly bed.
If you hadn't figured it out from the previous stanza, the speaker wants to clarify that the sleeping guys are not going to wake up. Here's how he explains it:
The first three lines of this stanza list different things that normally would wake a person up (at least, in the days before alarm clocks and cell phones). (1) The delicious smells of the breeze first thing in the morning ("incense" is a substance that you burn to make a room smell good). (2) Birds twittering and singing in their straw nests. (3) The rooster's cock-a-doodle-doo ("clarion" = "alarm"), or the echoes of a horn blown by a hunter or a shepherd.
Having listed all those things in the first three lines, the speaker tells us that none of those things are going to wake up the dead guys anymore. Okay, speaker!