Music appears universally in human infancy with self-evident effects: as many parents know intuitively, infants love to be sung to. The long-term effects of parental singing remain unclear, however. In an offset-design exploratory 10-week randomized trial conducted in 2023 (110 families of young infants, M = 3.67 months, 53% female, 73% White), the study manipulated the frequency of infant-directed singing via a music enrichment intervention. Results, measured by smartphone-based ecological momentary assessment (EMA), show that infant-directed singing causes general post-intervention improvements to infant mood, but not to caregiver mood. The findings show the feasibility of longitudinal EMA (retention: 92%; EMA response rate: 74%) of infants and the potential of longer-term and higher-intensity music enrichment interventions to improve health in infancy.