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.
I found that the funny moments made the rest of the game more intense. The dichotomy between a turret shooting at you and then saying something seemingly kind like “I don’t blame you” is funny, creepy, and kind of scary for me. Additionally, which most of GLaDOS’s dialogue is really funny (particularly at the end) the odd match up of emotions and actions made the game more intense and real for me. If it were just pure horror, then the game would have less impact on me, but because of the quick transitions between emotions, the game drew me into it more than a regular game would have.