This observation is especially true when dealing with a male school-age child.
The reason for this voice (or really speech) change is because the upper airway is much larger and more resonant after the tonsils and adenoids are removed.
Just like a person's voice sounds different when talking in a closet versus a large auditorium, the larger opening through which the voice travels after surgery is much larger than before and as such, the voice/speech has to correspondingly change after surgery as well.
The change is also much more dramatic the larger the tonsils and adenoids are.
SO, do not be alarmed if your child sounds different after surgery, especially if the tonsils and adenoids were extremely large. It is actually an expected occurrence and not an indicator of anything bad, swelling, healing, etc.