What is the output voltage range of computer headphone 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.