However when recording a good amount of data is being shuffled through ram, the cpu has to encode the wavestream, if stored the flash memory will have to do it's thing, if transmitted, the radios will have to wake up and run through all the data as well. The microphone does not exist in isolation.
This is not as power intensive as you would think though. If designed well these simple operations could last year's on a single battery cell. Most of cell phone battery use comes from transmitting and screen usage. Passive microphone and storage is nothing