Which fragments could grow into a piece of music with the ones already in a musician's open project? This target has no direct labels at cold start, and audio similarity is only one observable signal for it. Pocket Producer, a retrieval prototype evaluated offline, treats this missing-label problem as a ranking and an interaction-design challenge. It fingerprints the open session and ranks candidates with a rules default and a selectable fusion of five interpretable signals trained on pack co-membership as cold-start weak supervision. On the Freesound Loop Dataset, the learned fusion's apparent advantage varies with label construction, negative regime, and representation/training configuration. Pack co-membership is only a stand-in for the real target, and the conclusions shift with how the proxy is built and evaluated. We therefore surface retrieval as proposals the musician can inspect, preview, insert, and undo, not as automated decisions.