Phonemes, like letters, are divided into two categories: vowels and
consonants.  Loosely defined, a vowel is a continuous sound made with the
vocal cords vibrating and air exiting the mouth (as opposed to the nose).
A consonant is any other sound, such as those made by rushing air (like S
or TH), or by interruptions in the air flow by the lips or tongue (B  or
T).  All vowels use a two letter ASCII phonetic code while consonants use
a one or two letter code.

In English we write with only five vowels: a, e, i, o, and u.  It would be
easy if we only said  five vowels.  However, we say more than 15 vowels.
Narrator provides for most of them.  Choose the proper vowel by listening:
Say the word aloud,  perhaps extending the vowel sound you want to hear
and then compare the sound you are making to the sounds made by the vowels
in the examples on the phoneme list.  For example, the a in apple sounds
the same as the a in cat, not like the a in Amiga, talk, or made.  Notice
also that some of the example words in the list do not even use any of the
same letters contained in the phoneme code; for example AA as in bottle.

Vowels are divided into two groups: those that maintain the same sound
throughout their durations and those that change their sound.  The ones
that change are called diphthongs.  Some of us were taught the terms long
and short  to describe vowel sounds.  Diphthongs fall into the long
category, but these two terms are inadequate to fully differentiate
between vowels and should be avoided.  The diphthongs are the last six
vowels listed in the table.  Say the word made out loud very slowly.
Notice how the a starts out like the e in bet  but ends up like the e in
beet.  The a, therefore, is a diphthong in this word and we would use EY
to represent it.  Some speech synthesis systems require you to specify the
changing sounds in diphthongs as separate elements, but narrator takes
care of the assembly of diphthongal sounds for you.