+T to Gordon.
I'm not speculating on this one. But I bet most agree the microphone capsule is the part that matters most. My overarching view is that the most difficult, challenging and rewarding things to get right are those which involve conversion from one medium to another. As applied to an audio recording and reproduction chain, the hardest parts to get right are the transducers at both ends along with any conversions made by the transmission/storage-medium in between. Manipulations within a single medium might still be challenging but are easier. Electrical aspects such as buffering, amplification, impedance balancing, filtering, etc. all lie within the electrical domain.
The first line of my TS post signature reflects those critical conversions between different domains. Here's the core of it expanded, excluding the most critical performance/experiential parts at either end!
Microphone capsule converts variations of sound pressure to variations of voltage.
ADC converts voltages to numbers.
/Tape head converts voltages to magnetic flux.
/Cutter converts voltages to a squiggly engraving.
/Radio transmitter converts voltages into radio waves.
DAC converts numbers back to voltages.
/Tape head converts varying magnetic flux back to voltages.
/Cartridge converts squiggly engraving to voltages.
/Radio receiver converts radio waves into voltages.
Speaker drivers convert voltages into variations of sound pressure.