One might remember the iconic music system from Left 4 Dead, where the music would change depending on the context of current events. For example, high-tempo and loud music would play when intense actions was occurring in the game world, and vice versa. The music system has been redesigned in Portal 2, according to Morasky, that results in the “musical experience [tending] to become one of exploration, as if the music is emanating from the facility and devices of Aperture Science itself”. He notes that additional pieces of music will come from the device that the player is using in the present game play mechanic, such as a puzzle, and this can lead to a heightened sense of achievement within the player.
It is because of this that Morasky sees the mechanics of a puzzle turn into an interactive musical experience as the player progresses through it. This is achieved by “selectively triggering the different channels of music with differing timings and configurations”. It is assumed that this is a huge step forward from the music cues in Left 4 Dead (e.g. musical cues when reviving a fallen teammate) and Team Fortress 2 (e.g. vocal cues when killing an enemy), because Chell, the protagonist in the Portal series, is mute (much like Gordon Freeman) and the protagonists in the cooperative campaign are also, to an extent, mute. He also comments that the music has a positional factor to it, so that as a player moves through the game world the mix and volume of music will change, inviting players to explore the game world. During initial development, the team knew that they wanted some form of “musical humour, surprise and interactivity”, and the idea of making the game a music puzzle experience was even put forward, but after player-testing the team sought fit on the developing it in a very organic fashion “by identifying opportunities and challenges through team collaboration and testing”.
On the subject of player-testing, Morasky notes that the feedback from music in Portal is not really there to provide as much data as it did in Left 4 Dead. However, they did notice that during initial play-testing, players were showing puzzle fatigue (much like combat fatigue, a term that Valve uses often). The music was then developed to center around the puzzles, to make them appear fun and off [sic] in an “otherwise dark and threatening environment”. The players would feel happy and confident in completing them, suppressing or delaying fatigue from attempting to solve the puzzle in the environment of Aperture Science. It would “[help] lighten and energize the puzzle space without forcing so much music on the player as to distract them from the problem at hand”, and it is because of this that Morasky feels the music servers to expand the story and context of the game world rather than just accompany it.
Of course, music can add further context to events in a game. For example, if a friendly character is killed then one would expect sad and slow music to play that aims to enhance the emotions being driven in the player from the experience. Morasky wants to toy with this notion by asserting emotions that directly oppose the ideologies and connotations that are presented by the world of Aperture Science, in order to minimize the overshadowing of the dark and depressing environment over the puzzles that the players have to solve. In the original Portal, slow and dreary music would play regardless of where you were in a puzzle, and this has changed with the sequel, providing continuous audible feedback as the player progresses through the puzzle.
Morasky believes that because the experience of Portal 2 is different in many ways then that of the original Portal, an entirely new soundtrack had to be developed. He explains that the original Portal has two distinct music styles; the test chambers “had a sort of future of the 70’s vibe” whereas when the player progressed beyond the test chambers into the heart of the Aperture Science testing facility, the music changed to more of a “future of the 80’s style”. Morasky feels that Portal 2 covers a broad range of different kinds of music for different moods, but still carries forward the “future of the past (modernized)” musical theme that the original Portal game established. The development team did experiment with having old music tracks from Portal play in test chambers, but apparently is was considered confusing so the team dropped it.
Be sure to read the entire article over on GamesRadar to learn about the musical styles embodied by the soundtrack, and listen to the samples presented by Morasky to get a sense of what he is hoping to accomplish with the music in Portal 2.