It as true that normal people couldn't hear Gaspode speak, because dogs don't speak. It's a well know fact.
...
Besides, almost all dogs don't talk. Ones that do are merely a statistical error, and can therefore be ignored.

“Some of the men and older boys were trained in firing the cannon, but since the…”

“The politics of wizardry were either very simple, and resolved by someone ceasi…”

“Probably went swimming and got eaten by a pineapple.”

“Oh. I see. People don't want to see what can't possibly exist.”