It is true that the portal sounds really make the game what it is, particularly GLaDOS and her conniving, insane ramblings. One particular thing I want to focus on is the way the sounds set the mood or tone of the game.
Portal could easily be a horror game. Remove the robot’s, almost cute, questions, remove GLaDOS’s whimsical, comical ramblings and add some grinding noises, threatening gurgles and screaming. Take away the heart on the companion cube and add a skeleton and viola! A bonafida horror game.
The developers made very specific choices when it comes to the sounds of portal and these choices set the mood of the game. There are definitely creepy elements but the ridiculous things that GLaDOS says and the robot’s asking you if you are still there take the edge of the mystery and add and element of humor to the suspense.
This mood set by the music matches the skin and functionality of the game. The companion cube has a heart on it, the robots are cute little white eggs. The walls are also white and there is plenty of light in most of the levels setting a mood of “science” and “testing” rather then horror and suspense.
The functionality of the game matches this mood. There is something comical, not horrible about flying through the air or being stuck in an infinite loop between the floor and the ceiling. The graphics, sounds and functionality of the game all match perfectly.