The voice morphing process presented in this paper is based on the observation that, in many styles of music, it is often desirable for a backing vocalist to blend his or her timbre with that of the lead vocalist when the two voices are singing the same phonetic material concurrently. This paper proposes a novel application of recent morphing research for use with a source backing vocal and a target lead vocal. The function of the process is to alter the timbre of the backing vocal using spectral envelope information extracted from both vocal signals to achieve varying degrees of blending. Several original features are proposed for the unique usage context, including the use of LSFs as voice morphing parameters, and an original control algorithm that performs crossfades between synthesized and unsynthesized audio on the basis of voiced/unvoiced decisions.