DIY Frequency Response mic?

I wondered if a regular PC mic can be used to mesaure requency response and if theres any cheap software out the to do it? Also do I fix the output wattage on the amp measure the power then adjust it to the right voltage then leave it for the hole response recording?

You will find very few mics flat out to 20khz (especially of axis) unless you want to spend a few $k. If you can test the mic or get one that comes with an actual f response plot that was measured (not generic to the model ) you can use it to determine your speaker response. (you also need to get rid of the room effects)