Right, there is this PcmWaveWriter which can be used to wrote the Speex encoded data into a WAVE file. Using it is just straight forward. First, you construct an instance of the PcmWaveWriter using the longer contructor:
public PcmWaveWriter(int mode, int quality, int sampleRate, int channels, int nframes, bool vbr)
on this instance you call
WritePacket(byte data, int offset, int len)
for every packet encoded with Speex.