1. A speech recognition system, comprising: a processor operatively coupled to at least one memory to provide: a feature determination module configured to extract features from a speech signal within an environment; a condition determination module configured to determine at least one environmental conditioning parameter, wherein said at least one environmental conditioning parameter reflects a characteristic of the environment that affects a quality of speech recognition; and a speech recognition module configured to use variable-parameter hidden Markov models (VPHMMs) to recognize speech based on the extracted features and said at least one environmental conditioning parameter, each of the VPHMMs using a plurality of mixture components L in modeling state observations, and a particular mixture component l of the plurality of mixture components L having at least one mixture component parameter that varies as a function of said at least one environmental conditioning parameter, as approximated by using a piecewise function that includes a plurality of connected segments.