Speex dev - Feb 2010 - WAVE_FORMAT_SPEEX

Hi Alls,
I'm newbie in speex and try to use ACM speex in my application
What is way for initialization WAVEFORMATEX struct for speex ?
I tried this 
ZeroMemory(&m_Format, sizeof(WAVEFORMATEX));
unsigned int nBytesPerSample = 8;
m_Format.wFormatTag        = WAVE_FORMAT_SPEEX;
m_Format.nChannels        = 1;
m_Format.wBitsPerSample    = 8;
m_Format.nBlockAlign        = (nBytesPerSample*m_Format.nChannels);
m_Format.nSamplesPerSec    = 8000;
m_Format.nAvgBytesPerSec = m_Format.nSamplesPerSec*m_Format.nBlockAlign;
m_Format.cbSize            = 0;

But O got WavIn:20 error
Any idea ?
Thanks


PS
Sorry for my english