arXiv:2610.32211v1 Announce Type: new Abstract: Controlling melody in audio generation normally requires training a conditional generative model on audio paired with pitch curves or MIDI notes. We instead guide an existing unconditional diffusion model, originally trained without melody inputs, to follow a target melody without retraining or modifying it. During inference, a differentiable pitch detector steers the sampled audio toward the desired pitches, while the diffusion model keeps it close to the distribution of realistic audio learned during training. Our main contribution is a sampling method that balances these two directions, providing melody control while preserving audio quality. To improve stability, we initialize sampling with a simple synthesized version of the target melody rather than pure noise. Experiments on piano, guitar, and singing voice show melody accuracy comparable to conditional models trained specifically for melody control. Our method therefore adds melody control to existing audio diffusion models without requiring paired melody data or task-specific retraining.