Dallas — Want to have some fun tonight as you watch the Republican convention? Listen to the music - it's one of the control points for political excitement at such political gatherings.
For example, watch how quickly the applause for a speaker fades once the music after the speech ends. No music, no applause. Generally.
Of course Ronald Reagan's appearance tonight to accept his party's nomination for the presidency may be an exception. It is the high point of an unsus-penseful week here.
But in most cases, a politician can be momentarily boosted or let down gently by the way the music is handled. If you want to generate an atmosphere of excitement after a speaker finishes, you keep the music going a little longer.
''The music plays a more important part inany type of function than the average individual realizes,'' says Manny Harmon. He ought to know: He's been the bandleader at seven Republican conventions, including this one.
Sometimes the choice of music becomes a matter of political study. Often it is important to know a politician's favorite music.
It is the latter bit of political intelligence that makes it very likely you will hear the theme from the movie ''Rocky'' whenever Harmon is playing for Gov. George Deukmejian of California.
Altogether, Harmon knows more than 700 pieces of music. And he has made a point of learning the favorite pieces of Presidents, and sometimes those of their wives.
President Reagan's favorite piece, he says, is the theme song from the movie ''Dr. Zhivago.'' But Harmon plans to strike up a more presidential tune when Reagan makes his appearance tonight: ''Hail to the Chief.''
The choice of music sometimes has more implications than is apparent.
For example, on one evening of the convention here, two possible Republican presidential contenders for 1988 got slightly different treatment, musically speaking.
US Rep. Jack Kemp of New York got ''New York, New York.'' Kansas Sen. Robert Dole, on the other hand, got ''Happy Days Are Here Again,'' a rousing piece often associated with Democrats, and probably better known. (The latter piece was also played when Jeane Kirkpatrick, nominally a Democrat, appeared.)
''We wanted to make it a big deal,'' said Harmon, explaining the Dole music as he relaxed on the bandstand between numbers. Maybe it was intended to ''show the Democrats we're not afraid to play it.''
Who wanted to make it a big deal? Harmon wears a headset that keeps him cued to a ''production manager,'' who suggests when to start and end the music, and which music to play. (Harmon says he frequently makes his own decisions, however , especially as to when to stop.)
The production manager is nonpolitical, as far as Harmon knows. The manager simply has a view of the speaker's podium, something Harmon can see from his location on the convention floor only by watching the television screen beside him.
Not being able to see the people at the podium somtimes creates challenges. When, for instance, the national anthem is sung by a soloist, the television camera crews frequently cut away to shots of the audience. So Harmon has to lead the band on, just hoping the singer is keeping up.
The choice of music for keynote speaker Katherine Ortega, a Hispanic, took some time, says Harmon. ''We thought about it four or five days.''
''We were going to use 'Granada,' '' but that was rejected as being Spanish, and she is not from Spain. A song that could have been tied to divorce was also rejected. Finally they decided to use ''It's Today'' from the Broadway show ''Mame.''
And for some reason, ''we used to play 'Donkey Serenade' at Republican gatherings, even though their symbol is the elephant,'' says Harmon.
Harmon, a devoted baseball fan, was born in Philadelphia but now lives in Los Angeles. Between Republican conventions he leads his own band, with the big-band sound. Sometimes he conducts as many as nine performances; some weeks, none.
After Dallas, his next engagement is at a dental convention. There will be a lot less fuss over which tunes to play, he says.
But tonight he'll be up on the raised platform of the band, behind the delegates, out of view of the podium, but making his presence known with the help of a microphone stationed at each instrument. Only three members of the band travel with him from city to city; the others here are local.