Chell’s design perfectly shows how different the moods of portal 1 and portal 2 are. Portal 1 is much darker and bleek with things made in hast for mass production and cost. While searching for a quote to answer an other question, i read this passage in portalgame. wikia article about chell.

Why was chell woken up before everything got ruined? I was watching a playthrough of portal 2 and i totally forgot that she is briefly woken up by the announcer in the room before going back to sleep. In portal 1 she's definitely mute by choice, since we do have voice files of pain to play when she's hit by a cube for instance. In portal 2 though, there isn't even one of these. Initially in the scene where …

Initially in the scene where …