Voice leading is considered to play an important role in the structure of Western tonal music. However, the explicit voice assignment of a piece (if present at all) generally does not reflect all phenomena related to voice leading. Instead, voice-leading phenomena can occur in free textures (e.g., in most keyboard music), or cut across the explicitly notated voices (e.g., through implicit polyphony within a single voice). This paper presents a model of proto-voices, voice-like structures that encode sequential and vertical relations between notes without the need to assume explicit voices. Proto-voices are constructed by recursive combination of primitive structural operations, such as insertion of neighbor or passing notes, or horizontalization of simultaneous notes. Together, these operations give rise to a grammar-like hierarchical system that can be used to infer the structural fabric of a piece using a chart parsing algorithm. Such a model can serve as a foundation for defining higher-level latent entities (such as harmonies or voice-leading schemata), explicitly linking them to their realizations on the musical surface.