Involuntary musical imagery (INMI) is a ubiquitous cognitive phenomenon. The present study comprises six intensive interviews that examine the subjective phenomenology of INMI for individuals of varying musical experience. Grounded theory analysis was used to establish themes regarding both the form and feelings ascribed to INMI episodes. All the respondents felt that their INMI experiences had high fidelity including some complex musical arrangements. INMI form could be unstable over time according to cognitive demands. Individuals with musical experience reported concurrent sensations of visual and motor imagery as well as involuntary extemporizations of their INMI. Motivators for positive and negative appraisals of INMI were identified such as nostalgia and entertainment, and repetitive cycling with loss of control, respectively. We discuss interpretations from auditory memory theory and develop hypotheses for future INMI empirical investigation.