Acoustic analysis of vocal expression offers a potentially inexpensive, unobtrusive, and highly sensitive biobehavioral measure of serious mental illness (SMI)-related issues. Despite literature documenting its use for understanding SMI, prior studies have largely ignored that vocal expression is highly dynamic within individuals over time. We employed ambulatory vocal assessment from SMI outpatients to understand links between vocal expression, SMI symptoms, and affective states. Vocal samples were analyzed using a validated acoustic analysis protocol. Overall, vocal expression was not directly related to SMI symptoms but changed as a function of state and state by symptom interactions. The results suggest that (a) vocal expression fails to modulate across changing affective states in individuals with active SMI symptoms, (b) this lack of modulation may be commonly associated with many SMI symptoms, and (c) vocal analysis can accommodate temporal dynamics. (PsycINFO Database Record (c) 2019 APA, all rights reserved).