What is responsible for the whistling sound that often accompanies fireworks?

The design of the tube and the chemicals used.

Step-by-step explanation:

Tube design:

The firework tube is long and narrow one. Because the tube is narrow, the mixture of chemicals doesn’t burn so quickly that it will explode.

Furthermore, half of the tube is empty, so it behaves as a resonance chamber, like an organ pipe, and interacts with the expanding gases to make the whistling sound.

Chemicals used:

Often, the firework contains alternating layers of oxidizer and potassium benzoate.  

When the oxidizer and the potassium benzoate mixture burns, a layer at a time, the sharp explosions cause pressure changes in the gases. A standing sound wave forms in the tube, and this causes the whistle.

As the mixture burns away, the length of the resonating chamber increases. The increasing wavelength of the sound waves creates a descending whistle sound.