What became of the Mouth of Sauron?

[Keeper of Dol Guldur]

Actually, his weezing and choking in rage was him having a hyperventilation attack. He fled back to the gates and the various forts, and probably got dizzy, couldn't feel his hands and feet, and then suffocated.

Poor guy, I guess that's what happens when you eat crow, feathers and all. Especially Stormcrow, the most bitter of all the crows you could possibly eat.