Off the top of my head, if this is for radio, it would seem the use of sound effects would be imperative.  So, the script would not just contain dialog, but also sound effects, and perhaps  narrative.  Any run time would have to include them, as well as music, and commercial breaks, if any.  You also have to know how long the air time available to you is.

Is this your first attempt at such a project?

“Music is harmony, harmony is perfection, perfection is our dream, and our dream is heaven”
Henri Frederic Amiel