I don't know what a CA buzzer is....
My first surprise is when I measured the current. Tester in AC mode says the current is about 0.1 mA!
I wonder if that's true... What kind of "Tester" do you have? Are you sure it's not 0.1 Amp?
2) Why PIN8 is giving me so low current? Is there any way to increase it? Theoretically output PINs can load up to 40mA. Ohm's Law
You don't "push current". You put-out a known voltage, and the amount of current depends on the voltage and the resistance/impedance. (i.e. resistance is "resistance to current flow".)
So I thought, ok, lets improve the current adding a transistor. I used a 20year-old BD139. Base connected to PIN8, Emissor to GND, Collector connected to the buzzer, and buzzer connected to 5V. Still worst! I almost cannot hear any sound.
The transistor needs to be connected to a higher voltage. The 5V from the Arduino (through a current limiting resistor) into the base of the transistor turns-on current flow through the collector & emitter of the transistor, and through you buzzer.
It's the higher voltage that increases the current, and the transistor basically protects the Arduino from the higher voltage, whichoud damage it if you connect it directly.