I believe "vehement" is the correct answer. It means that it shows strong, powerful, passionate speech and tone.
Whitman wanted to express that America is a country constituted by several parts that make a whole that works well. Each individual of the country doing their part is what forms the nation.
A choir is a group of people in which each one has a part to sing that might not be special on their own but form a harmony when they are sung together,