21. A non-transitory computer-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 a plurality of different types of phonetic sources of pronunciation difficulties between a language of a non-native speaker and a second language, said determining including determining multiple types of sounds in the second language that cause pronunciation difficulties; assigning a weight to each source of pronunciation difficulty, the weight representing a measure of severity of the source of pronunciation difficulty; calculating using a processor a phonetic difficulty score, wherein calculating the phonetic difficulty score comprises calculating a product of a number of times the phonetic source occurs in an utterance of the non-native speaker and the weight of the phonetic source; calculating using the processor a language model score based on a sound comparison between the language of the non-native speaker and the second language, wherein the language model score is calculated based on a difference between a cross-entropy of the utterance with respect to a language model of the language of the non-native speaker and a cross-entropy of the utterance with respect to a language model of the second language; normalizing using the processor the phonetic difficulty score and the language model score; and calculating and outputting a metric from the normalized phonetic difficulty score and the normalized language model score, the metric providing an assessment of pronunciation difficulty.
21. A non-transitory computer-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 a plurality of different types of phonetic sources of pronunciation difficulties between a language of a non-native speaker and a second language, said determining including determining multiple types of sounds in the second language that cause pronunciation difficulties; assigning a weight to each source of pronunciation difficulty, the weight representing a measure of severity of the source of pronunciation difficulty; calculating using a processor a phonetic difficulty score, wherein calculating the phonetic difficulty score comprises calculating a product of a number of times the phonetic source occurs in an utterance of the non-native speaker and the weight of the phonetic source; calculating using the processor a language model score based on a sound comparison between the language of the non-native speaker and the second language, wherein the language model score is calculated based on a difference between a cross-entropy of the utterance with respect to a language model of the language of the non-native speaker and a cross-entropy of the utterance with respect to a language model of the second language; normalizing using the processor the phonetic difficulty score and the language model score; and calculating and outputting a metric from the normalized phonetic difficulty score and the normalized language model score, the metric providing an assessment of pronunciation difficulty. 23. The non-transitory medium of claim 21 , wherein calculating the language model score comprises calculating a language model for the language of the non-native speaker and the second language.