The text to announce for speech tracks. Can either be a format string or a function.
If it is a function, it should return the format string to
announce. The function is called for each audio event, and
receives a parameter object with time, and potentially
point and value properties depending on the track.
point is available if the audio event is related to a data
point. value is available if the track is used as a
context track, and valueInterval is used.
If it is a format string, in addition to normal string
content, format values can be accessed using bracket
notation. For example "Value is {point.y}%".
time, point and value are available to the format
strings similarly to with functions. Nested properties can
be accessed with dot notation, for example
"Density: {point.options.custom.density}"