Mixed Reality (MR) technologies have recently been explored in many areas of Human-Machine Interface (HMI) such as medicine, manufacturing, entertainment and education. However MR sickness, a kind of motion sickness is caused by sensory conflicts between the real world and a virtual world. The purpose of this paper is to find a method to reduce MR sickness based on MR technology with a Head-Up Display (HUD). This MR environment is modeled after advanced driver assistance systems in near-future vehicles. In this paper, a relationship between auditory stimuli and equilibrium function after whole-body vibration related to MR technologies is evaluated. The seated subjects in the MR simulator are shaken in the pitch direction at 0.2 and 2.0 Hz. Consequently, it is shown that auditory modulation is useful for diminishing postural instability. Moreover, we discuss what type of auditory stimuli is most efficient to reduce MR sickness by changing the sound level and exposure timing.