Okay, I might hit a time machine button but honestly first three pages didn't show results so apologies for raising this topic up twice haha
On one hand, whatever the magic and wherever insides go, there are fewer of them or they are larger when inflating, right? So since the spaces are wider, the tone produced by voice glands (is it glands in English?) should be much much lower. May be as lowest male voice or even infrasound. Kinda like when the poor birdie competes with Fiona in Shrek her voice when she is so round and stretched and big is much lower than her regular voice
On the other hand, if we don't want our beloved inflatee squish us flat with a couple of tons weight, we won't go for air for inflation and will stick to Heliox or a similar composition - or just to plain good ol'Helium. And Helium voice does sound high pitched. More helium = more pitch
That's marvel physics approach. There's also "what you like most" approach. Alexi Strigoii once wrote a story where pitch was reflecting fullness
Finally, there's tradition. We accumulated a fair amount of big data to fetch statistics from, how, in general, inflated people talk compared to their uninflated speech
What do you think?
I am a big fan of squeaky helium voice. I wish it came up more in inflation stories. And while it doesn't really make sense, I like the idea that more helium = higher pitch, so the inflatee keeps getting squeakier.