finding microphone specification
The sensitivity is -38 dBV for a 1 kHz sound pressure level of 94 dB (the data sheet tells you that) and -38 dBV is 12.6 mV RMS so you now know the signal level in volts, the frequency (1 kHz) and the sound pressure level (1 pascal = 94 dB SPL).

If the sound pressure level decreased by 10 dB the voltage output would fall by 3.162 to 4 mV because 10 dB is a ratio of \$10^{10/20}\$.

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