WOW!
if you no matter to change the pitch when you increase or decrease the speed, you can just change the sample rate !
Can be very simple using python:
import wave
CHANNELS = 1
swidth = 2
Change_RATE = 2
spf = wave.open('VOZ.wav', 'rb')
RATE=spf.getframerate()
signal = spf.readframes(-1)
wf = wave.open('changed.wav', 'wb')
wf.setnchannels(CHANNELS)
wf.setsampwidth(swidth)
wf.setframerate(RATE*Change_RATE)
wf.writeframes(signal)
wf.close()
increase or decrease the variable Change_RATE
!
Now if you need keep the pitch untouched, you need do same type of overlap-add method !