13. A non-transitory processor-readable storage medium for assessing pronunciation difficulties of a non-native speaker, comprising computer-executable instructions, which when executed cause a processing system to execute steps comprising: determining one or more sources of pronunciation difficulties between a language of a non-native speaker and a second language, assigning a weight to each source, calculating a phonetic difficulty score based on the one or more sources and the weight assigned to each source, calculating a language model score based on a sound comparison between the language of the non-native speaker and the second language, by calculating a language model for the language of the non-native speaker and the second language and calculating a cross-entropy of an utterance with respect to the language model of the language of the non-native speaker inversely weighted by the cross-entropy of the utterance with respect to the language model of the second language, wherein calculating the cross-entropy of an utterance with respect to the language model of the language of the non-native speaker comprises assigning a lower score to utterances of the second language that are similar to sounds of the language of the non-native speaker, and assigning a higher score to utterances of the second language that are not similar to sounds of the language of the non-native speaker, normalizing the phonetic difficulty score and the language model score, and calculating a metric from the normalized phonetic difficulty score and the normalized language model score.
14. The non-transitory processor-readable storage medium of claim 13 , wherein calculating a language model for the language of the non-native speaker and the second language comprises phonetically transcribing the first language of the non-native speaker and the second language by applying a phonetic dictionary to text representing recorded utterances of each language.