With the rapid growth and popularity of YouTube, an increasing number of consumers rely on online product review videos to obtain product-related information. As the provision of online review videos grows and consumers increasingly rely on them for their purchase decisions, understanding factors that contribute to the perceived helpfulness of video reviews becomes critical for video review management. This paper examines how various visual and vocal characteristics of online review videos are associated with the perceived helpfulness of videos. We collect detailed observational data on 13,840 electronic product review videos posted on YouTube and employ video content analysis, speech recognition, and natural language processing techniques to extract the visual and vocal characteristics of review videos. By using econometric models, we find that the increase in visual stimulation, captured by brightness and visual dynamics, increases the perceived helpfulness of reviews. In addition, featuring reviewers' faces in review videos increases the perceived helpfulness of videos. Consumers also perceive review videos in which reviewers express more positive facial emotions as more helpful. Furthermore, lower voice pitch and faster speech rates are associated with higher perceived helpfulness of reviews. To complement the empirical analysis and further isolate the causal effects of review brightness and pitch, we conduct controlled experiments. Overall, the findings can facilitate the management and operation of online review videos for product reviewers, businesses, review platforms, and consumers. In particular, the findings provide direct and actionable guidance to content generators who aim to create more helpful product reviews.