Speaking rate relative to the default rate for this voice. 1.0 is the
default rate, normally around 180 to 220 words per minute. 2.0 is twice as
fast, and 0.5 is half as fast. This value is guaranteed to be between 0.1
and 10.0, inclusive. When a voice does not support this full range of
rates, don't return an error. Instead, clip the rate to the range the
voice supports.