GScholz, thank you very much for investigating this.
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There's one issue from above that hasn't been discussed:
Why can the guns be heard so well, and not the engines?
Directional microphones is one reason. Some mics can screen out most sound from all but a few degrees. I don't know what the level of directional mic technology was in 1943, but as someone said above, "it wasn't the stone-ages then".
Also, by plugging into the intercom system background noise would be screened out.
MRPLUTO