Believe it or not, there is a Standard American English which speech-language pathologists are taught in college in the US. The International Phonetic Alphabet was established in the late 1880s. It was first concerned with phonetics as applied to teaching English, but as more countries became involved, the alphabet became truly international and has survived to this day with very few changes.
As you know, the same letters can have entirely different sounds. For instance, the "c" in city
makes the "s" sound and the "c" in cage
makes the "k" sound. The IPA has a symbol for each sound in our language. Although we use only 26 letters in English, there are (dredging up very old memory) 44 symbols that represent the different sounds or phonemes in Am. English. The different dialects we hear are simply variations on a theme that have been established regionally. They occur because of changes in voice quality, vowel distortions or prolongations, omissions or additions of sounds, inflections, and the influences of other languages, among other things. The phonetic alphabet has modifying symbols to show these variations.
Oh, for pity's sake, I sound like one of my professors. Sorry! Class dismissed!
Jeremy Brett was also my favorite Sherlock Holmes. Holmes' speech should be that of an educated Victorian Londoner. Anything else would be jarring.