Sidious was a Sith Lord. He sounded just like Senator/Chancellor Palpatine. They had the same nose, mouth and chin. Palpatine had actually been the actual name of the Emperor as far back as a comic book or strip around the beginning of the '80s although his name never made it on-screen until the prequels came along. The book or strip spoke of a girl or Imperial palace subject "who grew up at the feet of the Emperor Palpatine" on the capital planet of the Empire.
Scratch all that, man. Ian McDiarmid played the "Emperor" in Return of the Jedi
back in 1983. If you were paying any attention (people who were children in 1999 obviously excluded) it was painfully obvious who he was supposed to be in The Phantom Menace
My only question at the time was who was the actual Phantom? The secret Darth Sideous guy (who was obviously McDiarmid), or the secret Darth Maul guy who had five lines? Lucas did a shitty job of making that clear. (And now I remember the Yoda line at the end of the movie, so yeah. But still.)