Those who are physically dependent on opioids and benzos take very high doses. With increased amounts, they become lethargic and will have slurred speech along with other side effects. When someone is on low doses of a benzo, they don't slur their speech, but the desired effect of decreased agitation can be witnessed. IMO, Sergio's affect was grossly inappropriate for the situation. He should have been agitated. He also misinterpreted that the 911 operator wanted his wife's height and not his daughters and he had difficulty spelling Isabel's name. Perhaps he is normally "goofy" like this, but I doubt it.
Maybe he actually has a prescription for Xanax or Klonopin? That would hardly be an unusual thing as lots of doctors hand those out like candy.