It is generally well-known, and scientifically well established, that music affects emotions and moods. However, only little is known about the influence of music on thoughts. This scarcity is particularly surprising given the importance of the valence of thoughts for psychological health and well-being. We presented excerpts of heroic- and sad-sounding music to n = 62 individuals, and collected thought probes after each excerpt, assessing the valence and the nature of thoughts stimulated by the music. Our results show that mind-wandering emerged during listening to either type of music (heroic, sad), and that the type of music strongly influenced the thought contents during mind-wandering. Heroic-sounding music evoked more positive, exciting, constructive, and motivating thoughts, while sad-sounding music evoked more calm or demotivating thoughts. The results thus indicate that music has a strong effect on the valence of thought contents during mind-wandering, with heroic music evoking more empowering and motivating thoughts, and sad music more relaxing or depressive thoughts. These findings have important implications for the use of music in everyday life to promote health and well-being in both clinical populations and healthy individuals.