We struggled initially with the various choices of symbols to use: IPA common, IPA used in Australia and China ( some slightly different symbols), Americanist Phonetic Notations (APN) symbols, or a combination of the first three. Different dictionaries, ESL texts, linguists and teachers seem to use slightly different symbols as well focusing on the different sound blends which gives us anywhere from the 44 to 56 "sounds" for the English language.
Working closely with the Linguistic department of the University, the choice of symbols was deliberate and was motivated by the feeling that learning the phonetic alphabet is often an unnecessary task for most learners of English - and one which may even impede (or at least interfere with) their progress.
Note that we have, on the top right hand corner, used the symbol of the sound that the student is studying on each page. The user may click on that symbol at any time to be reminded of the sound they are working on. Also, the program features the "toggle" button where the student can highlight the sound that they are working on in all of the words, thus helping them recognize the various ways that certain sounds may be spelled.
Please note - http://en.wikipedia.org/wiki/Pronunciation_respelling_for_English