It is the summer months in the strip right now, making this feasible. I can't help but think that big hole overhead must affect the acoustics somehow.
One interesting thing about Peanuts' art style is how the characters' mouths disappear when closed. It's particularly evident on Schroeder's face here, since he doesn't speak in this strip.
When viewed from the front, the characters' mouths have generally been visible up until now, even if only as a short line. We'll see in the years to come that Schulz plays around with this a bit, that there will be times when characters seen from the front will strangely have no mouths.
