Ok, I will try 60Kohm and hope for the best.
But, this leaves a lot of questions open for me. I am sure I am wrong on many points here so if someone could help me understand what is going on, I would appreciate it.
As far as I understand, if I reduce the imdedance (move from 1 Mohm to 1 Kohm), then not as much current will flow through the piezo into the monitoring pin and thus my readings will be lower. But this makes it a bit strange to me. If I an doing analogRead, aren't I reading how much current goes into the pin? If that is true, then how does the ground line to grounded Arduino pin read anything?
Also, I don't really understand why having more piezos would change the required resistor value. If 1 Mohm is fine for 1 piezo, why is it too much for multiple piezos and why if I now find the correct resistor value, would I not have to change it if I add more piezos?