There are different configurations for the New Orleans marching band. Often saxophones are used as well as the traditional clarinet, trumpet and trombone configurations. In the earlier forms, the most recorded has been with banjo accompaniment. There was the outdoor parade style of trad jazz and the indoor variety with piano, guitar or banjo, sometimes violin, drums and the usual clarinet, trumpet and trombone and occasional saxophone. Both musical forms were used for dancing. The marching bands had the "second line" whereby the listeners could march alongside the musicians or dance. The indoor variety were sponsored by whore houses..one notably Lulu Whites in the Storyville District of New Orleans named after Alderman Story and was closed down by the Navy in 1913(?).

To answer the question, the trombone plays a tenor line that employs a lot of dominant sevenths. The clarinet, thirds and the trumpet, melody line but these are not hard and fast rules. The banjo plays mostly chunk chunk on the beat. The piano plays a kind of stride stsyle and very important is the New Orleans press roll on the drums. Zutty Singleton, Barbarin, and others play in that style.

The trumpet takes the lead (melody). The clarinet fills with obligato lines interweaving and the trombone, a tenor "descant", in the traditional style not too much going on but punctuating the rhythm with growls, smears and strong attacks. Often, the band will play together in a chordal style which has been worked out in advance as an arrangement. A model would be the "Tin Roof Blues" (not an original New Orleans tune but adapted from an older blues) where the clarinet plays the third, trumpet, root and bone, flatted-seventh.

The so-called dixieland style taken from the first recording of jazzs by the Original Dixieland Jass Band in 1914(?) was not in the strictest sense a traditional New Orleans jazz band but an adaptation and popularization set the model with trumpet on the melody, clarinet above and trombone below. They did not use a banjo but a piano and drums. The New Orleans Rhythm Kings eventually recorded with banjo using somewhat the same "front line".

This is probably more than you needed to know but a listen to these bands would give you an idea on how dixieland units sound.

Frank