In 1929, the United States motion picture industry released more than 300 sound films — a rough figure, since a number were silent films with music tracks, or films prepared in dual versions, to take account of the many cinemas (not yet wired for sound).
rough = approximate
a number = a number of them = a number of the films = some of these films
take account of sth = take sth into account = include sth in your considerations
not wired for = not ready/prepared/equipped for …
many cinemas (that were) not yet wired for sound
在1929年，美国的电影行业发行了300多部有声电影
不过这只是个大概的数字，不是准确的数字
因为其中有些电影是（配有音乐的）无声电影或带有无声及有声两个版本的电影，
【它们不是严格的有声电影】
这么做是考虑到了当时很多电影院还没有足够先进的设备来播放有声电影。
bird老师解答vip学生Luna