Audio producers often use musical underlays to emphasize key moments in spoken content and give listeners time to reflect on what was said. Yet, creating such underlays is time-consuming as producers must carefully (1) mark an emphasis point in the speech (2) select music with the appropriate style, (3) align the music with the emphasis point, and (4) adjust dynamics to produce a harmonious composition. We present UnderScore, a set of semi-automated tools designed to facilitate the creation of such underlays. The producer simply marks an emphasis point in the speech and selects a music track. UnderScore automatically refines, aligns and adjusts the speech and music to generate a high-quality underlay. UnderScore allows producers to focus on the high-level design of the underlay; they can quickly try out a variety of music and test different points of emphasis in the story. Amateur producers, who may lack the time or skills necessary to author underlays, can quickly add music to their stories. An informal evaluation of UnderScore suggests that it can produce high-quality underlays for a variety of examples while significantly reducing the time and effort required of radio producers.