The index of the current character in the utterance. For word events, the
event fires at the end of one word and before the beginning of the next.
The charIndex represents a point in the text at the beginning of the
next word to be spoken.
The length of the next part of the utterance. For example, in a word
event, this is the length of the word which will be spoken next. It will
be set to -1 if not set by the speech engine.
The type can be start as soon as speech has started, word when a word
boundary is reached, sentence when a sentence boundary is reached,
marker when an SSML mark element is reached, end when the end of the
utterance is reached, interrupted when the utterance is stopped or
interrupted before reaching the end, cancelled when it's removed from
the queue before ever being synthesized, or error when any other error
occurs. When pausing speech, a pause event is fired if a particular
utterance is paused in the middle, and resume if an utterance resumes
speech. Note that pause and resume events may not fire if speech is paused
in-between utterances.