This paper studies the application of voice conversion to hearing-impaired Mandarin speech enhancement. The system is based on the combined use of a sinusoidal analysis-synthesis model and a priori knowledge about Mandarin syllable phonetic structures. We propose a time-scale modification algorithm that finds accurate alignments between hearing-impaired and normal utterances. Using the alignments, spectral conversion is performed by a continuous probabilistic transform based on a Gaussian mixture model. Simulation results indicate that the proposed system can improve the intelligibility of hearing-impaired Mandarin speech.