Mercutio is saying that he is dying, and he blames the feud between the Montagues and the Capulets. Just before he dueled with Tybalt, and his famous line was "a plague on both your houses" as he cursed both the Montagues and Capulets. Basically, he is referring to this curse here, when he says "your houses.". Mercutio is upset because Romeo got in the way of his fight with Tybalt, and caused his injury. He knows he is dying. When he says, "I have it, and soundly too" you can think of it as "I've had it! I've really had it!" He is saying that he is dead, and he blames Tybalt and Romeo, and the feud, so the Capulets and Montagues in turn.