Some follow-up references:
The Hooven report states that "...Naru radio reported that it had heard the same voice on the same frequency the next day, 0831, 0843, and 0854, after the flight had ended, but too weak to distinguish words. "The same voice
without the hum of the plane in the background."" (emphasis mine)
The TIGHAR Wiki description of the radio
including a picture of the microphone can be found here. The mic has the appearance of a non-noise cancelling design, but the back is not shown, so it is not proven one way or the other.
Michael Everette's Technical Analysis of the Electra Radio can be found
here. It includes the statement that "...Additionally, the audio fidelity was poor, mainly due to the microphones employed. Carbon microphones of the 1930s and 40s were very similar in design and construction to a telephone-type microphone element. These microphones had to be close-talked; that is, held up almost directly in front of the lips.
They were not noise-canceling, so any background noise from the aircraft engines and propellers was also picked up, further reducing intelligibility. It was necessary to speak slowly and deliberately, usually with a raised voice, to make oneself understood."
The comment about "...held up almost directly in front of the lips" might imply that Fred would normally be too far away to be heard intelligibly. Then again, if there was no background noise, perhaps he could be heard. That implies a relatively quiet cockpit. How quiet would it have been with engines idling?