I was given a microphone spec:

• Vout (RMS) = 0.25mV
• Zout = 9.5 Ohms

And I wish to use an audio generator to simulate such microphone. This can be accomplished by a voltage divider with the right resistor values.

Given the low voltage on the output as well as the low impedance, we could have a hunch that R2 should be very close to 9,5 ohms, but let's do some math.
Note: Zout on my generator is predominantly resistive, hence I've added Z and R without further ado.
Choosing 1V(RMS) for our source (that we can control anyway) and a Zi of 40 Ohms and solving the matrix we have:

No surprise here, but the math might be very useful to simulate other sources.

On the scope of this project I should use a 9,1 Ohm resistor (or even a 8,2 Ohm) for R2 and a 39KOhm for R1 and adjust my Vo to have .25mv on the output.