We know that frequencies of sound waves varies from 20Hz to 20,000Hz, so its corresponding wavelength varies from 15m to 15mm respectively. The size of slit (almost) becomes comparable to wavelength of sound so diffraction of sound wave takes place easily.
But the wavelength of visible light varies from 0.4 to 0.7micron which is very small. So the size of most of the slits is not comparable with wavelength of visible light, due to this diffraction of light cannot take place.