He knew the foe he triumphed o’er The name of great Márícha bore. “The fiend,” he pondered, ’ere he died, “Ho, Lakshmaṇ! ho, my Sítá!” cried Ah, if that cry has reached her ear, How dire must be my darling’s fear!

In plain words: Rama realizes the creature was Maricha. He worries that if Sita heard the demon's cry in his own voice, she must be terribly frightened.