Max input voltage to soundcard for use with Visual Analyzer?
I'm thinking that I should certainly be safe to at least 1 - 2 volts p-p


For a microphone input: certainly far too high (think more in the range of 10-20 mV pp); you risk damage.

For a line-in input, this might be OK, if still much too high.

In any case: you'll need some knowledge of what the input impedance of your sound card is, and match your voltage source's impedance to that, otherwise you'll get bad measurements.