Analog reading with noise - Arduino Uno VS ESP12e
The ESP8266 is an incredibly (electrically) noisy environment. 

Because you can't use anything except the internal voltage reference, which is in turn driven from the internal noisy power, you can't get anything even vaguely approaching stable readings.

It is recommended to disable the WiFI completely ("Modem Sleep") when doing ADC readings to reduce noise.

However, the ADC was never intended for this kind of work. Its primary role is for monitoring the battery voltage of your hardware - and as such it really doesn't care that much about noise, since you can average a number of samples for greater accuracy (something you can't really do with audio).