How can I make an AC analog voltage with Arduino?
You have a few options:


Use a low-pass filter to round the square wave's edges some so they look more AC-ish.
Use a low-pass filter and have your code create steps of voltage that look AC-ish by varying the PWM widths.
Use an external SPI DAC chip that you can control with more discrete levels to make smoother looking AC.
Look into Direct Digital Synthesis (DDC) to learn more. Google "analog via ddS", lots of hits.