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; access information defining viseme units, the viseme units comprising animations that simulate lip movement of an animated entity, the viseme units including a first set of viseme units that simulate lip movement for one or more complete phonemes and a second set of viseme units that simulate lip movement for one or more partial phonemes, an individual partial phoneme spanning one of a beginning portion, a middle portion, or an end portion of an individual complete phoneme without inclusion of a remaining part of the individual complete phoneme; determine potential sets of viseme units that correspond with the first phoneme string portion, wherein different ones of the potential sets of viseme units form different viseme strings that define different animations of lip movement to simulate 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.