That only happened in relatively early strips, with a disembodied speech balloon coming from off-panel. For the vast majority of the strip’s time, adult speech is not represented. When it’s necessary to know what an adult has said, the kids will repeat it. “What’s that, Miss Othmar? Why isn’t my book report done? Well, you see, it’s like this…”
This was certainly well established by the time they started making the TV specials, which I suppose is why they came up with the device of the muted trombone.
And the Easter Beagle was also Snoopy! Good grief, it’s Snoopy all the way down, isn’t it?