JD-EE,
Am equally baffled by any suggestion of IRU problems.
Hot microphones
Can't find what RR_NDB said right now. Can tell you, though, that the clearest signal on the BAC 1-11 CVR that I heard in 1980/81 was from the hot mikes. Also occurs to me that if you play back the P1 & P2 mics simultaneously, it may provide a stereo effect. Don't know how well the mics pick up distant noises, but am confident that it's good enough to hear a pilot talking even when his headset is lying on his side console. I have a feeling that hot mikes for CVRs were advocated in the UK from the start, whereas other states were not so keen.
"Co-pilots"
This is not a pejorative term; nor does it imply inferior training (or even experience). It's a description of a role in the cockpit. To keep it simple, an A320 or B737 on a routine short-haul line-flight has two pilots, of course. The pilot-in-command is referred to as the "pilot", or P1. He has been nominated as such prior to the flight, and signs the Tech Log and loadsheet. The other pilot is the "co-pilot", or P2. When the captain "gives" a sector to the co-pilot in an enlightened airline, the captain will take over the co-pilot duties, and let the P2 take over the routine command tasks and decisions. But he/she still carries the can. We could discuss this for hours, and I don't want to teach granny to suck eggs. So I just hope this helps.