This section of "Song of Myself" is a commentary on the many fallen heroes that fight battles for us in war, and never come home to the victory parades, bands and marches that are thrown in honor of returning armies. Whitman mentions how he wants to come with "cornets and...drums" to "beat and pound...and blow" not for the men who came home, not for the "victors," but for the
"slain persons...the dead...those who have fail'd...who sank in the sea...overcome heroes..."
What he discusses, raising the happy bands and parades for the fallen, is a poignant point to make; unfortunately, it is the fallen men and women of war who never hear the praises and accolades sung for them. The meaning of the poem can also be applied to areas outside of war; for anyone who has ever suffered failures and trials, for everyone who has struggled and had a hard fight. Whitman, a man who feels a connection to everyone in life, no matter their station or success, wants to sing the praises of all.
I hope that those thoughts helped; good luck!