Head-Mounted Displays (HMDs) are increasingly used to support mobile work (Laramee & Ware, 2002). Human operators sometimes require additional auditory support when using an HMD, which raises the question of whether sound is better delivered publicly in free-field or privately via earpiece. A novel experimental procedure was created in which participants had to identify mismatches between auditory information and visual information on an HMD. Different conditions of sound delivery and physical movement were manipulated within-subjects. Participants heard the sound either via earpiece or free-field while they either sat or moved about the test room. Predictions were based on the idea that inconsistent spatial mapping of vision and sound would compromise mismatch detection. First, I predicted a main effect of movement such that participants‟ mismatch detection would be worse when they moved than when they sat. Second, I predicted an interaction between movement and sound delivery. When participants are seated there will be no difference in mismatch detection between the two methods of sound delivery. When participants are walking, however, mismatch detection will be better with an earpiece than with free-field delivery. Results supported the first prediction. For the second prediction, the significant interaction found took a different form than predicted. With the earpiece, participants performed equally well whether sitting or walking, but with free-field sound, participants performed better when sitting than when walking. Results have implications for understanding necessary auditory conditions for effective crossmodal integration and may indicate a cause for concern for people who use HMDs and auditory displays in safety-critical environments.