Immersion is an essential technical feature of immersive virtual reality (VR) environments, which can affect various learning and psychological outcomes. However, limited research has studied the impact of immersion via multiple mechanisms. In this study, we investigated the effects of immersion in a VR science learning environment using two different mechanisms, delivery format and sound stimuli, on various cognitive and psychological outcomes, including learning, cognitive load, self-efficacy, presence, enjoyment, and usefulness. Through a 2 (delivery format: immersive VR vs. Desktop VR) x 2 (sound stimuli: yes vs. no) design, one hundred and twenty participants experienced one of four versions of a VR tour on nature-based science learning, including an immersive VR (iVR) tour with or without sound, and a desktop VR (dVR) tour with or without sound. Both quantitative and qualitative data were collected. Results indicated that dVR groups rated significantly higher than the iVR groups on perceived learning, presence, self-efficacy, and usefulness, regardless of sound stimuli. However, neither immersion mechanism impacted knowledge retention, cognitive load, or perceived enjoyment. Meanwhile, all groups significantly improved self-efficacy after their condition-dependent VR experience. Qualitative data from participant responses provided additional perspectives on the quantitative findings. This research fills a gap in the limited existing literature by investigating immersion through multiple mechanisms in VR learning environments. The findings offer both theoretical and practical implications for researchers and practitioners.