Diegetic Sound: is generated by things we can see in the picture. It is sound that is heard by the character in the film. Things like trees rustling, wind, standing on leaves and a song on a radio.
Non-Diegetic Sound: Raw footage is usually enhanced through the addition of non-diegetic sound, usually in the form of music or voice over.
Soundtracks: Previously-recorded music chosen because of its fit to the rhythm, content and mood of a sequence. If a song, the lyrics may add meaning to the film.
Voice over: The most efficient way to record a voice over is to find the quietest room possible and speak directly into the camera microphone. You can either record your own voice or find someone who suits the position better to do it. Remembering the gender, age and status of the person doing it will have different effects on the trailer/film its recorded for, as all of them will have implications for the meaning of the text used.
Sound Effects: Sometimes you just have to fake the sounds within the frame. Sound effects are often used to heighten realism by adding in sound at a crucial moment. Often you can download free sound effects to fit within the trailer.