Marmite (pronounced "marmeet") is the French word for a large round cooking pot, usually made of iron. By extension, marmite had come to be used as a slang word for a heavy artillery shell that resembled a round iron cooking pot.

Therefore "when the marmites are a-soaring" makes perfect sense and is parallel in meaning to "while the batteries are roaring" two lines before, just as "I yarn of fur and feather" is a parallelism for "I brag of bear and beaver".