There are numerous techniques use in composition for video game, with varying complexities. Some attempt to mirror player actions whereas other choose to serve as a bed of sound void of any intentional relation to player autonomy.
Non-Interactive Forms and Educated Guesses
The purpose of the music in Halo: Combat Evolved during the level ‘The Maw’ is non-interactive:
Although this is the case the music still connects with the player during this suspenseful part of the game, contributing to induce certain emotions, there is also the use of a stinger at 00:05:00 to aid this, yet it does not sync with the music.
Pneuma (2015) by Deco Digital:
Although the music plays in a looping fashion with little melodic content it does transition to new pieces of music when entering new areas. Whilst walking up the stairs at the beginning of the prologue to the game it could be argued that an educated guess was made to play the harp in an increasing scale at a moment when the composer felt the player may be just about reaching the top near the door. After playing this through myself and intentionally taking longer to arrive at that point the music does start this harp sequence and this continues even if standing still. To create an interactive element to this, the developer could have triggered each note for every set number of stairs the player moves up, ending in a crescendo when arriving at the top, much like it does when the user in this play through reaches the button or in my play through where this happened part way up the stars.
The transition from one piece of music to the other usually with a basic fade in fade out system, this can be heard here:
This could have been done in a more effective way with the use of parallel forms or horizontal re-sequencing. This would have created a greater flow to the composition had less impact on breaking user immersion.
Parallel Forms and Ornamental Forms
A basic example of parallel layering (vertical re-orchestration) can been seen here:
The game Kameleon (2015) by Kajero demonstrates this as well as the use of musical stingers (ornamental form):
The music changes the deeper the player goes into the water, using different instrument loops that are added and subtracted from the initial bed of music. This is also mirrored if they player returns to the top.