In this study, we used near-infrared spectroscopy (NIRS) to examine the effects of environmental sounds on performance of a serial arithmetic task. Subjects included 6 males and 15 females aged 21 or 22 years. All subjects were required to perform a serial arithmetic task according to the Uchida-Kraepelin performance test. We used four environmental conditions: hubbub sound, forest sound, traffic noise, and a silent control condition. During the serial arithmetic task, we also measured hemodynamic changes in the frontal cortex using NIRS to assess the effects of environmental sounds on brain function during the serial arithmetic task. Results showed that exposure to environmental sounds did not enhance or detract from task performance speed in a serial arithmetic task performance when compared with results obtained upon exposure to silence. However, environmental sounds enhanced cortical responses during the serial arithmetic task. Our results reveal differences in activation of the prefrontal cortex under different sound conditions, which may help increase our understanding of the potential effects of environmental sounds.