// byte[] dataIn -> byte[] dataOut
// convert to short
short[] data = new short[dataIn.Length / 2];
Buffer.BlockCopy(dataIn, 0, data, 0, dataIn.Length);
byte[] dataOut = new byte[dataIn.Length];
int encSize = speexEncoder.Encode(data, 0, data.Length, dataOut, 0, dataOut.Length);
if(encSize > 0){
    // encode success
    // todo with dataOut[0-encSize]
}

// byte[] dataIn -> byte[] dataOut
short[] data =  new short[1024];
int decSize = speexEncoder.Decode(dataIn, 0, dataIn.Length, data, 0, false);

if(decSize > 0 ){
    byte[] dataOut = new byte[1024];
    Buffer.BlockCopy(data, 0, dataOut, 0, decSize);
    // todo with dataOut[0-decSize]
}