Benna Su.
Part2b)
This is the photo of my Low pass filter circuit.

2c)This is my picture of my high pass filter circuit.

Part3) Adding a switch!

Schematics!

Part4)
I think you have to make the resistor or capacitor smaller in order to build the filter with 12db octave or 24db octave.
part5)
a) For me, I want to build a filter with high pass, low pass and band pass. And also I want to add a distortion on my filter, so it can add distortion while it passes out the sequence at the same time.
b)If I could, I would build a synthesizer with the function of adjusting ADSR, frequency modulation, filtering, etc together. Basically I want it build as a mini moog, you can play melody while you adjusting different elements.
c)I am not sure but I think Microphone should definitely be one of the analog electronics equipments that I want to study and learn the function of it. I know it might be pretty difficuilt to build a good mic, but if i have a chance, I will definitely build one for myself.