What is the technical term of the opening theme that played before the part in a film in which the name of actors and actresses are displaying?
The main title is the music, often later recorded on soundtrack albums, that is heard in a film while the opening credits are rolling. It does not refer to music playing from on-screen sources such as radios, as in the original opening credits sequence in Touch of Evil. A main title can consist of a tune sung by the leading character over the credits, such as Moon River, sung by Audrey Hepburn in Breakfast at Tiffany's, or the main orchestral theme as written by the composer, such as the famous The Pink Panther Theme. It can also be a medley of themes from the film, as in the 1959 Ben-Hur. In the film A Hard Day's Night, the title tune was heard over the opening credits showing The Beatles running from their fans. An overture may serve as a main title, as in The Sound of Music. However, there is a very strong difference in a roadshow theatrical release between an overture and a main title. The overture in such films is heard on pre-recorded tape or film, before the film even begins, while the house lights are still up and there is yet no picture on the screen. The main title begins when the film actually starts. In the case of The Sound of Music though (and also Fiddler on the Roof), no overture was heard before the lights in the theatre went down; therefore, in these cases, the main title did serve as an overture. Both films had pre-credits opening sequences; during these, the first song from the film was sung, and immediately afterwards, the main title music followed.
In movie musicals, the main title nearly always consists of one or more of the songs from the film, played by the orchestra. Wiki
The first two tracks on the Lawrence of Arabia CD are Overture and Main Title.
The the CDs for the films Alien, All That Jazz, and Silence of the Lambs, for example, all begin with the track Main Title.
Names of actors (and others) displayed on the screen may be called titles. Music played with them may be called title music. Titles the beginning are opening titles. Music played with them would be opening title music.
title music NOUN
Music played during the credits at the beginning or end of a television programme or film.
It seems this is in the UK section at Lexico, but not in the US section.
It is called theme music
Theme music is a musical composition that is often written specifically for radio programming, television shows, video games, or films and is usually played during the title sequence, opening credits, closing credits, and in some instances at some point during the program. (Wikipedia)
Filmsite defines the term as
the opening or closing music of a motion picture, often containing the film's 'signature' or leitmotif tune/phrase that is associated with a character or situation within the film.
So the fact that it is played before the film, does not mean that it will not be used during the film too, for specific purposes.
Learnersdictionary also records theme song:
a song that is played at the beginning and end of a television show, movie, etc. He was playing the theme song from/to “The Godfather.”
I have seen it described as the main title