Whilst i won't accept this as the answer i'd like to point out that after further tests the last code sample below seems to return PCM data. I made a waveform visualisation and the data returned includes values range from negative to positive in a wave like structure which can be passed into an FFT window and then into an FFT calculation.
// audio file reader
reader = new Mp3FileReader(filename);
byte[] buffer = new byte[reader.Length];
int read = reader.Read(buffer, 0, buffer.Length);
pcm = new short[read / 2];
Buffer.BlockCopy(buffer, 0, pcm, 0, read);
