1. A signal-bearing medium tangibly embodying a program of machine-readable instructions executable by a digital processing apparatus of a computer system to perform speech recognition operations, the speech recognition operations comprising: detecting at least a target word known to an acoustic vocabulary but unknown to an embedded grammar of a language model of the speech recognition system; assigning a language model probability to the target word; calculating a sum of an acoustic and language model confidence score for the target word and words already included in the embedded grammar of the language model; and if the sum of the acoustic and language model probability for the target word is greater than the sum of the acoustic and language model probability for the words already included in the embedded grammar, adding the target word to the language model.