If a Radio or TV needs to be audible. How is that written in a Script? Is it written like the characters, or is written in the description in parentheses?
I'd write it in the script. You have to hire someone to read the lines, and it's audible dialogue which the characters and audience have to hear and react to.
I've got the tea. Where are the biscuits?
Upper cabinet to the left of the sink, bottom shelf.
And now, we present for your enjoyment the dramatization of Neil Gaiman's Neverwhere, with James McAvoy as Richard Mayhew, Natalie Dormer as Door, David Harewood as the Marquis de Carabas, and Benedict Cumberbatch as the Angel Islington.
All right, all right! You'd think the man's never heard a radio drama before.
If ever thou gavest hosen or shoon,
Then every night and all
Sit thou down and put them on,
And Christ receive thy soul...
Number 13 on this page explains that
...if there’s a TV, and a reporter is talking, and your character is engaged with it, you need to provide all the dialogue.
However, if its just background noise:
Yes, there’s such a thing as walla. That’s background noise, the indistinct chatter of others in public places. You don’t have to write that in. But you do have to write in anything in which your protagonist or character is engaged, and would therefore hear ‘in the real world.
So, it appears if it's significant, it should be written like any other dialogue in the script.
The script for Night of the Living Dead - available here as a pdf - treats audible dialog from a television like dialog from the other characters (see pp. 5-6)