Designing a safe Amplifier to drive Sound Card Mic Input
The mic-in of consumer sound cards is designed to power (DC bias) and amplify the signal of an electret condenser mic. Alas there's no real standard how to do this. 

Ignoring the [usually 1.5-2.5V] bias voltage which you can just block with a capacitor (for your application), the input impedance also varies a fair bit between products, but you can expect it around a couple of kiloohms or even lower, but probably no less than 600ohms. The default amplification will be somewhere around 100x (40dB), but some cards offer a boost function of around 20dB (another 10x) or even more, although I think the latter is typically done in software.