On balance, probably yes he did, but Warner Bros. are sticking to their story that he didn't. Dark rumours abound that his parents were 'gotten to' and the actor has never spoken about it again, even as an adult.
Sowerbutts claimed (in an interview with The Sun newspaper) that he was present at the studio for a single morning and was told that he was doing voiceover work that would be included in the film.
"I was only there for one morning at Shepperton Studios but it was great fun. I couldn't believe it when they told me I was going to be
in the film. I was really surprised that Daniel's voice broke because
he's younger than I am. But you could say it was a lucky break for
me." Joe added: "I'm a massive Harry Potter fan and I've read all
the books. Being Harry Potter was so cool. My friends, brother and
sister are really jealous. Everyone is like, Wow, you're playing Harry
Potter. It's unbelievable. However, I'm only in it a bit. I've got two
scenes; one where I'm flying around on a broomstick and another when
I'm in the forest."
The Sun - Nov 1st 2001
When asked to confirm this, the studio's spokeswoman rushed out a statement to the effect that they didn't use Sowerbutts' voice in the final cut and that Daniel Radcliffe was the sole voice of Harry Potter.
"Despite what has been reported, only Daniel's voice is used in the
forthcoming film. Do you think we would be that silly? Production of
the sequel is under way and it is absolutely Daniel's voice in that as
Scottish Daily Record - Nov 2nd 2001
"No voice other than that of Daniel Radcliffe's was used to portray Harry Potter in the film Harry Potter And The Philosopher's Stone."
Daily Telegraph - Nov 2nd 2001
Although the two statements aren't mutually exclusive (since voiceover actors are often used in the rough cut and then replaced by the original actor using ADR) he and his parents were evidently reminded of the dire consequences of speaking without studio clearance and haven't spoken about it since, which strongly suggests that we'll never know the whole truth of what happened...
Julie Sowerbutts: "We have to accept what Warner Bros are saying and offer a no comment. I've got a little boy who wants to be an actor long term and I'd hate to wreck anything for him. If Joe's voice was to be in the first film then it was only going to be a couple of lines. Maybe they didn't use it."
Scottish Daily Record - Nov 2.