Soul Reaver (1999) was a game rather ahead of its time. The team at Crystal Dynamics wished to create a seamless open world that was free from loading screens; an ambitious idea for the time, which meant the music had to match. Due to the Playstations use of CD's it meant that audio could now be streamed from sampled files; rather than an in-built synthesiser with a very limited amount of audio channels. The problem in the case of Soul Reaver was that simply streaming tracks from the CD wasn't a seamless process, as there would be a break in the audio when the tracks switched.
To remedy this, Kurt Larson, game audio professional and member of the synth-pop band Information Society, created multiple variations of instrument tracks that would mute and unmute themselves depending on the games state .
Audio programmer, Jim Hedges, explains just how this worked:
"We used an in-house developed adaptive audio MIDI driver, which replaces the Sony driver entirely. Signals from the game, based on location, proximity and game-state set special music variables, which are read by the driver and used to effect changes in the MIDI data. How these signals are interpreted is controlled by an extensive scripting language with standard branching, logic and arithmetic functions. This scripting language is written using MIDI text 'meta' events. These text commands can be written in a standard text file, or interspersed with other MIDI data in the MIDI bytestream. Some of the changes to MIDI data available are: muting/unmuting, transposition, pitch mapping, sequence start/stop, volume/tempo/pan changes etc.
As an example, in Soul Reaver, every piece of music in the game has several arrangements which correspond to different game states. The default arrangement, or "ambient" mode, is used when no signals from the game are present. When the player comes within range of an enemy (wether seen or not), a signal is sent to the driver which sets a designated "danger" variable. The script sees this change in the variable and mutes/unmutes tracks to produce a more intense "danger" arrangement. When the player engages in combat, another variable signifying combat is sent, and the same process ensues, this time with a tempo increase. If the player stops fighting or kills the enemy, the combat variable changes again, and this time certain tracks from the "combat" arrangement begin to fade out. If the player resumes combat, they fade back in. If no combat resumes, the combat tracks fade out entirely and the music changes back to either "ambient" or "danger" mode" .
Now the music was seamless with the open world. The adaptive process wasn't necessary for most games of the time as they tended to be level based with loading screens in between. This enhanced the experience greatly as any breaks in the music would have been a distraction for the player. Soul Reaver wasn't just a technical marvel, but also one of atmosphere and immersion. Where most games still had very upbeat "blippy" music, Soul Reaver was dripping with dark, electro-industrial mood music that was incredibly absorbing to listen to and perfectly complimented the twisted, desolate vampiric wasteland. In some instances the music sits in a region of equally diegetic and non-diegetic as it layers in distant screams and instrumentation that sound quite monsterous, bridging the void between the safety of the musical score and the imminent danger that the sound effects signal for the player .
Much like in film, the role of sound in games is to create believability and immersion in the visual experience. This is because of the added value, a term originally coined by Michel Chion, that the sound provides to the image. As Walter Murch states in the foreward of Chions' book 'Audio Vision':
"...whatever virtues sound brings to the film are largely perceived and appreciated by the audience in visual terms - the better the sound, the better the image." .
Game audio has the ability to take it one step further and become reactive to the viewers own experience, adding a new layer of depth that the film industry could only dream of. Soul Reaver was a large audio success that strived to break technical boundaries that met the requirements of the image. It is a great example of early immersive, reactive audio in the days before middleware programs like Wwise existed in order to create a full and unforgettable experience.
 Soul Reaver DF Retrospective - https://youtu.be/ni13xjHvwu4?t=15m7s  The Eidos Interviews - https://www.iasig.org/pubs/interviews/eidosjh.shtml  Soul Reaver OST - https://youtu.be/0HfYVKJEGvo?t=1h9m  Chion, M. (2013). Audio-vision. Armand Colin, p.viii.