Output voltage? Electret microphones are a form of condeser mic's...I do not believe they actually produce any form of voltage, but rather vary a charge across two plates. For electrets,this is generally amplifed by an internal FET. This is why they require a power supply.
'Output voltage' and 'normal voice' are subjective. The only way you're going to know if it is for your application is to knock up a test circuit, get an oscilloscope, and tlak into it in your 'normal voice'.
That said.......I have seen many preamplifiers that are designed to use an electret mic, and pass it to line level on audio equipment. Line level is usually around 1v peak-to peak. These preamps have gains ranging from 20 to 40dB. From that you can work out the typical input signal
A two-wires electret mic has an output pin which needs power and a ground pin that also connects to its metal case. It can drive an LM386 little power amplifier that has its gain set to 200 or make this preamp circuit for it: