As i had good experience receiving help last time i asked a question i will try again,

I have attached the circuit down below.

I'm supplying the circuit with 3.0V. I'm using a voltage divider because i want a precise value for my piezo speaker. I want my piezo speaker to receive 0.88V (is going to be fitted in a bigger circuit). My R1 is set to 100 Ohms, and my R2 to 42 Ohms. By doing this, i can measure 0.88V at the pin going to the speaker (when the speaker is NOT there). But when i insert my piezo speaker, and measure the voltage, it drops to around 0.546V.

How can this be? Has it something to do with the piezo speaker having a inner resistance or something like that?

 

Well the answer is simple. Your "piezo speaker" loading the voltage divider and this is why the voltage drop.
What type of a piezo do you use? Also why you want 0.88V across the piezo ?
For example I have two buzzers with gen and one has 2kΩ of a internal resistance
http://blog.vidadesilicio.com.br/wp-content/uploads/2015/05/piezo-buzzer.jpg

And the big one has 500Ω
http://squishycircuitsstore.com/Single_Components_Images/Squishy Components/Piezoelectric_Buzzer.JPG