Capacitor microphones, including electrets (which do not need external voltage to polarize the capacitor), can be modeled as a voltage source (from the acoustic input) in series with a not-too-huge capacitor.
To get decent response in the audio range, they require a high resistance load, in parallel with a capacitance much less than that of the microphone.
Without a JFET (or cathode follower) to buffer the actual load, the output will be disappointing.