Why did flashback Sméagol have the same voice as present day Gollum?

Ha ha, for sure! That always takes me out of the movie a bit.
Anyone know if it was written in the book that way? I get that it may not have been mentioned though. I'll get around to reading the book one day but haven't gotten around to it yet.

haha yeah i laughed thinking, poor guy was always fucked

Perhaps the flashback is supposed to be more of a “dream” with the events being depicted as a condensed version of how they actually occurred and how Gollum sees his past self in his own mind.

Immediately thought that in yesterday’s rewatch, why on earth did Sméagol sound like Gollum five hundred years before the War of the Ring….

The scene wasn't very long. But it was supposed to portray that Smeagol was weak of character. Hence the little, passive aggressive voice.

The voice is the only way we know it's Gollum. Otherwise no one would have a clue. It's just a way to show he's the same person as Gollum...