I don't have actual knowledge, but here are some factors to show that parts of it, at least, are certainly voice-tracked ahead of time.
1. Two jocks are on seven days a week.
2. The evening jock (6pm- midnight) does a live, local AM drive show in New York City.
3. The former outcue for the music minute at the bottom of the hour used to be, "at the bottom of the hour, give or take a minute." Sometimes the clock would really drift, and the music minute would be heard as early as :15 after or as late as :45 after.
4. The morning jock also does an evening shift on one of the Sirius country channels.