Voicing is the process of changing the tone or "voice" of a piano. Pianos that are too bright or tonally aggressive can be made more mellow, and likewise a piano that is muffled can be made more powerful and sonorous.
Voicing generally speaks about quality of tone. Volume and projection can be improved to a degree, but are not as inherently movable as tone quality.
Typical voicing procedures are: leveling and mating strings to hammers, using softener or lacquer solutions, filing hammers and needling hammers. Each has a separate effect on the tone of the piano and most often a combination of techniques are used.