I want to find out what the voltage range should be for a typical laptop microphone input. Should it be AC with negative voltages, or a sinewave that stays positive?

This is a screenshot from the datasheet of the Realtek ALC892 audio chip:

That specifies a voltage between -0.3 AND 3.6 V.

Tested on a laptop with a sine wave between 0 and 500 mV and this gave a good signal. Possibly it can go much higher but I did not test that.