Perform http request to get sound from given surah and ayah.

        Parameter:
            :surah      --  Surah index from API pages.
            :ayat       --  Ayat key.

        Return:
            :string     --  URL for mp3 sound.

def __get_sound(self, surah, ayah):
        """
        Perform http request to get sound from given surah and ayah.

        Parameter:
            :surah      --  Surah index from API pages.
            :ayat       --  Ayat key.

        Return:
            :string     --  URL for mp3 sound.
        """
        # Formatting ayah with 0 leading.
        # http://stackoverflow.com/questions/17118071/python-add-leading-zeroes-using-str-format
        format_ayah = '{0:0>3}'.format(ayah)
        sound_url = '{base}/sounds/{surah}/{ayah}.mp3'.format(
            base=self.BASE_API, surah=surah, ayah=format_ayah
        )
        return sound_url