Relationship quality is important for well-being and quality of life in couples living with dementia. Home-based music therapy interventions may be conducted with the aim of enhancing relationship quality. However, the effects or influences of such interventions are only briefly investigated in previous studies. This study’s aim was to identify how a 12-week home-based music therapy intervention may influence relationship quality in couples living with dementia, through an adapted convergent mixed methods design. In this case, 68 participating couples from the HOMESIDE RCT study, and four individually recruited couples, received the music therapy intervention. Relationship quality for all participants was measured by the standardized Quality of Caregiver-Patient Relationship scale, and qualitative interviews were conducted with the four individually recruited participants at baseline and post intervention. Quantitative analysis indicated no statistically significant intervention effect. However, relationship quality remained stable over the intervention period. The qualitative analysis identified that the music therapy interventions primarily led to positive emotions, closeness, intimacy, and communication between the persons with dementia and their care partners. Intervention influences could also be ambiguous, as sharing music experiences might involve a risk of evoking vulnerabilities or negative emotional responses.