Can sound be turned into electrical energy? Can we somehow transform the noise wave sounds, from a highway, to electrical energy?
 A: You could do this, but the amount of energy is too low to be useful.
Even if the noise from the highway was continuous and loud enough to cause long term hearing damage, the sound energy level would only be about 1 milliwatt per square meter. You would need several square meters of sensors even to power a single LED light, which isn't very practical.
Human hearing works on a logarithmic scale of loudness, not a linear one - a typical "noisy crowd of people" environment only has an energy of about 1 microwatt per square meter, and the threshold of human hearing is about 1 picowatt ($10^{-12}$ watts) per square meter.
See http://www.physnet.org/modules/pdf_modules/m203.pdf. 
A: You are doing it whenever you speak in a mic . There is a transducer in the mic that is converting your sound energy to the electrical energy which is then converted back to the sound energy through speakers. In your idea we will have to use a very big or a very high tech transducer which may costs a lot and is not economical. 
