1. A system for speech animation using visemes with phonetic boundary context, the system comprising: one or more physical processors configured by machine-readable instructions to: obtain phoneme strings, the obtained phoneme strings including a first phoneme string, the first phoneme string including a first phoneme string portion; determine potential sets of viseme units that correspond with the first phoneme string portion, a viseme unit comprising an animation that simulates lip movement of an animated entity, individual ones of the viseme units corresponding to one or both of one or more complete phonemes or one or more phoneme context of one or more complete phonemes, a given phoneme context including a partial phoneme that spans the beginning, middle, or end of a complete phoneme, wherein the individual ones of the potential sets of viseme units that correspond to the first phoneme string portion form different viseme strings that define different animations of lip movement corresponding to the first phoneme string portion, the potential sets of viseme units including a first potential set and a second potential set; and select one of the potential sets of viseme units based on a fit metric that conveys a match between individual ones of the potential sets and the first phoneme string portion.