Encode a PCM byte array using NSpeex

gistfile1.txt

// I call EncodeSpeech() with a byte array containing PCM-encoded audio data that I get from the 
// microphone (with len being the number of encoded bytes in the buffer, in case this is the 
// last chunk and the buffer is only partially filled).  
// In my app I send the returned byte array to a service for processing - hence I prefix the 
// encoded Speex data it with length information.
// The reason that I prefix the data with both the original PCM buffer size as well as the 
// the size of the encoded chunk is that the DECODER needs to know the original buffer size, 
// otherwise it does not decode properly.  Of course the size of the encoded chunk is to "frame"
// the chunks for the service, which may read multiple chunks off a stream.

        private static Microphone mic = Microphone.Default;

        private static byte[] EncodeSpeech(byte[] buf, int len)
        {
            BandMode mode = GetBandMode(mic.SampleRate);
            SpeexEncoder encoder = new SpeexEncoder(mode);
            
            // set encoding quality to lowest (which will generate the smallest size in the fastest time)
            encoder.Quality = 1;

            int inDataSize = len / 2;
            // convert to short array
            short[] data = new short[inDataSize];
            int sampleIndex = 0;
            for (int index = 0; index < len; index += 2, sampleIndex++)
            {
                data[sampleIndex] = BitConverter.ToInt16(buf, index);
            }

            // note: the number of samples per frame must be a multiple of encoder.FrameSize
            inDataSize = inDataSize - inDataSize % encoder.FrameSize;

            var encodedData = new byte[len];
            int encodedBytes = encoder.Encode(data, 0, inDataSize, encodedData, 0, len);
            if (encodedBytes != 0)
            {
                // each chunk is laid out as follows:
                // | 4-byte total chunk size | 4-byte encoded buffer size | <encoded-bytes> |
                byte[] inDataSizeBuf = BitConverter.GetBytes(inDataSize);
                byte[] sizeBuf = BitConverter.GetBytes(encodedBytes + inDataSizeBuf.Length);
                byte[] returnBuf = new byte[encodedBytes + sizeBuf.Length + inDataSizeBuf.Length];
                sizeBuf.CopyTo(returnBuf, 0);
                inDataSizeBuf.CopyTo(returnBuf, sizeBuf.Length);
                Array.Copy(encodedData, 0, returnBuf, sizeBuf.Length + inDataSizeBuf.Length, encodedBytes);
                return returnBuf;
            }
            else
                return buf;
        }

        private static BandMode GetBandMode(int sampleRate)
        {
            if (sampleRate <= 8000)
                return BandMode.Narrow;
            if (sampleRate <= 16000)
                return BandMode.Wide;
            return BandMode.UltraWide;
        }

Here's the code:
private byte[] DecodeSpeech(byte[] buf)
{
BandMode mode = GetBandMode(microphone.SampleRate);
SpeexDecoder decoder = new SpeexDecoder(mode);

        byte[] inDataSizeBuf = new byte[4];
        byte[] sizeBuf = new byte[4];
        byte[] encodedBuf = new byte[buf.Length - 8];            
        Array.Copy(buf, 0, sizeBuf, 0, 4);
        Array.Copy(buf, 4, inDataSizeBuf, 0, 4);
        Array.Copy(buf, 8, encodedBuf, 0, buf.Length - 8);

        int inDataSize = BitConverter.ToInt32(inDataSizeBuf, 0);
        int size = BitConverter.ToInt32(sizeBuf, 0);
        short[] decodedBuf = new short[inDataSize];
        int decodedSize = decoder.Decode(encodedBuf, 0, encodedBuf.Length, decodedBuf, 0, false);

        byte[] returnBuf = new byte[inDataSize*2];
        for (int index = 0; index < decodedBuf.Length; index++)
        {
            byte[] temp = BitConverter.GetBytes(decodedBuf[index]);
            Array.Copy(temp, 0, returnBuf, index * 2, 2);
        }

        return returnBuf;
    }

I am also facing issues while decoding. Can anyone share how to decode using Speex ?