How can we produce fourier transform of an electrical signal ??
I would also like to know whether fourier transform can be taken for mechanical waves like sound waves practically

 

You can form the Fourier Transform of any sequence of values.

You can do this on an oscilloscope that has the FFT function.

Or you can do this mathematically on a computer using any programming language.

 

I would also like to know whether fourier transform can be taken for mechanical waves like sound waves practically

An optical lens is capable of forming the FT of an incident light pattern, so by analogy an acoustic lens should work for sound waves. How practical that would be I don't know.

 

For frequencies in the audio range, you can use a software signal analyzer application such as SignalScope.