Setting volume has no effect

[cyplas]

(I'm using this library for the first time (thanks!), so I may be doing something wrong.)

All three sound files generated seem to have the same volume:

import tts.sapi
voice = tts.sapi.Sapi()
voice.set_voice("CUSTOM-HUNGARIAN-VOICE")
voice.voice.Volume = 0
voice.create_recording('test_silent.wav', u'görög ábécé')
voice.voice.Volume = 50
voice.create_recording('test_normal.wav', u'görög ábécé')
voice.voice.Volume = 100
voice.create_recording('test_loud.wav', u'görög ábécé')

I've tried voice.Volume instead of voice.voice.Volume, but same result.