voiceSetRate does not work (osX)

i try to modify pitch and rate for the voices (tts);

i have used the methods indicated in the scripting dicionnary

voicesetPitch(), voiceSetRate()

the text is correctly read but nothing changes when i try to modify the rate

when i ask "voiceGetRate" i allways get the same value, very very high (3538944)

if i try to change it to 3538943 the voice slows down in a brutal way and reading becomes very bad

the values indicated in the help (-10, 10) are obviously not the values i get

working with 11.5, osX 10.6.8

code example:

on  mouseUp(me)

    voiceSet(10)

  voiceInitialize()

  if voiceGetRate() >= 3538944 then

    voiceSetRate(3538943)

    put voiceGetRate()

  end if