User blog comment:Stormwell/Thoughts on Technology/@comment-1500935-20140210135550/@comment-3525200-20140210142925

There is a cable visible in the loudspeaker; http://static1.wikia.nocookie.net/__cb20120310100706/dishonoredvideogame/images/8/80/Loudspeaker.jpg

Besides, you don't have to route the cables through the sewers since they can easily be run via rooftops, etc.