The screenplay is not just a list of basic events and dialogue for your movie, but an actual roadmap that a director interprets on screen for an accurate film. Though it has to be open enough so that the filmmakers can initiate their own creative point of view, it also has to clearly convey the intentions of the screenwriter.
As events occur in the story space, a number of different things may occur. Actions will happen, sets will change, and there will even be indicated montage sequences. Sounds are another thing that are sometimes included in screenplays, but they have to be correctly formatted and chosen for inclusion.
The only time that a sound is included is when it is central to the scene that is in the screenplay. This can be something that indicates an action that is occurring, a characteristic of the environment, or even a unique sound made by one of the characters. If it is just a common-side result and not the focus of the scene, then it should just be determined by the director and not the screenwriter.
When you are adding these, they are put in all capital letters because it is supposed to be interpreted as important. You do not need to indicate that it is a sound since its separation from dialogue, scene description or actor’s direction should leave it clear what it is. Example:
Janie looks strait at Paul and POPS HER GUM.
These do not need to be in a specific place. But, if they are in a scene description, the capital letters should indicate that it is an important sound. If it is going to stand on its own, as with LOUD THUD, then it can be separated in between pieces of dialogue. Make sure that the situation is such that people can understand that the sound is stand alone and a unique part of the scene.
There is a nice balance that you should look to find between description and excessive detail. A spec script should not put too many details that will be discovered during the film’s production. At the same time, you need to include detailed sounds that are important to the character’s and story.