Evan is correct. You can get a nice S-meter working in the base CEC sketch by adding a small amplifier (Ian has a circuit and descriptions on his site) and feeding the aux input. I did that, it works well, and you can adjust the meter sensitivity using Memory manager. I later added the second Nano and circuit and changed to the -S sketch to get the small audio spectrum display and CW decoder, and it also changed how the S-meter signal is developed so the small amp and aux input are no longer used. The S-meter display is virtually the same in the 2 methods, although maybe a little faster in the I2C "-S" version.
=Vic=
Vic WA4THR