What would a 1940s audience have understood from the phrase "wired for sound"?

The question resolves most of its own issues.
Wiring motion picture theaters for sound had happened in the decade before WW2. That conversion was fresh in the minds of many.

What Irving Berlin might have meant about a bomb that's wired for sound was probably not much, except to get a rhyme with ground. Ariel bombs during WW2 were typically fin stabilized, with the air flowing over the fins creating something of a whistle. Falling bombs were not silent.
An additional wired for sound in WW2 was the German Stuka dive bomber. This plane itself was wired for sound. Anyone watching a sound newsreel might mistake the airplane's siren for its bomb. The notion of screaming death from above had been established by 1943.

This is a good question that mostly answers itself.