Firstly you are putting the rate before setting it to 1 so
the 2621440 is the rate before you set it.
That's the bit that seems weird as the docs suggest a rate
between -10 and 10. On my iMac I got an initial rate of 11796480
when I did the voiceGetRate() and setting it to progressively lower
values slowed the voice down. However I couldn't detect any further
slowing down below a very high threshold.
In short I have no idea what a correct value for the rate
should be (aside from the initial value).