Player immersion is the holy grail of computer game designers particularly in environments such as those found in first-person shooters. However, little is understood about the processes of immersion and much is assumed. This is certainly the case with sound and its immersive potential. Some theoretical work explores this sonic relationship but little experimental data exist to either confirm or invalidate existing theories and assumptions. This paper summarizes and reports on the results of a preliminary psychophysiological experiment to measure human arousal and valence in the context of sound and immersion in first-person shooter computer games. It is conducted in the context of a larger set of psychophysiological investigations assessing the nature of the player experience and is the first in a series of systematic experiments investigating the player's relationship to sound in the genre. In addition to answering questionnaires, participants were required to play a bespoke Half-Life 2 level whilst being measured with electroencephalography, electrocardiography, electromyography, galvanic skin response and eye tracking equipment. We hypothesize that subjective responses correlated with objective measurements provide a more accurate assessment of the player's physical arousal and emotional valence and that changes in these factors may be mapped to subjective states of immersion in first-person shooter computer games.