Every mic is different, the voltage typically depends on the resistor that you have before the mic. The larger the resistor, the lower the voltage. 300mV and 100uV is a significant difference, but they both still need a preamp. And dynamic mics should still have a preamp regardless of how robust it is. The preamp is increasing the signal level so that it can be heard easier. A preamp can be made using an array of transistors to boost the signal, or it can be an audio amplifier.