emucompboy
1. It may use up all the oxygen in the bottle before it completely explodes -- which would make it sound softer. 2. It may explode the bottle, in which case you'll find out firsthand what Lt. Henry Shrapnel invented. Advice: Don't do it. You're likely to injure yourself or someone else.

gman
The sound of the firework exploding will be muted. However, the sound of you screaming when the bottle explodes and fills you with shards of glass will probably be pretty loud.

stucco_bill
yes, it will be louder, since containing the expanding gases will fous the energy. course, you might want to have a wall between you and the bottle.

fizixx
Do you ever whisper to someone by cupping your hands to your mouth?

j h
No, but, you could put a microphone hooked to an amplifier turned full volume right next to the explosion, and that might make it louder.