Does graves blasting through still deal damage as an attacker moving in if my opponent responds to the ability with sirens Siren's Song

No — if your opponent uses Siren's Song to move Graves off the battlefield before his attack trigger resolves, Graves deals no damage to the units there.

That's because "here" in "deal damage equal to my Might to all enemy units here" is a referent checked at resolution, not at trigger (Rule 359.3.f.2 — same principle as Yasuo, Remorseful being moved in response to his attack trigger, where "here" no longer matches). Once Siren's Song moves Graves, "here" is wherever he landed (normally his Base), so the ability hits whatever enemy units are at his new location — almost always nothing. His Might is likewise read at resolution, so any Might change would also be counted then.

Two caveats worth checking. Siren's Song needs a unit that moved this turn, which a Standard Move into the contested battlefield satisfies, but a unit merely played from hand has not moved (Rule 448.2) — so if Graves entered some other way, the spell can't legally target him. And the trigger still resolves and is still "played" even with nothing to damage; it just does nothing.