I made a simple microphone with amplifier which outputs 9V max signal. It's biased in the middle somewhere around 4.5V. I hooked the output of the circuit to the computer speaker audio in. I was wondering how much it can take or what is the recommended input and output voltage ranges of computer headphones and speaker ports?
If it's a true microphone input, then it's looking for mV signals. A "line in" port usually wants around 1 V RMS. A "speaker" output port will be low impedance and have some power capability since it's intended to drive 4 or 8 Ohm speakers, usually to at least a few Watts. Anything else is hard to guess.