source: OniSplit/Sound/SoundData.cs

using System;
using System.Collections.Generic;
using System.Text;
using System.IO;

namespace Oni.Sound
{
    internal class SoundData
    {
        public bool IsPCM;
        public bool IsIMA4;
        public int SampleRate;
        public int ChannelCount;
        public byte[] Data;

        public UInt16 nGameTicks;

        public int SampleCount; // calculated 

        //MS WAV variables
        public int AverageDataRate;
        public int BlockAlignment;
        public int BitsPerSample;
        public int HeaderSizeADPCM;

        // optional MSADPCM values
        public UInt16 SamplesPerBlock;
        public UInt16 nCoefPairs;
        public Int16[] CoefADPCMa;
        public Int16[] CoefADPCMb;

        public static SoundData Read(InstanceDescriptor sndd, bool do_pc_demo_test)
        {
            if (sndd.Template.Tag != TemplateTag.SNDD)
                throw new ArgumentException("descriptor");

            var sound = new SoundData();

            int dataSize;
            int dataOffset;

            using (var reader = sndd.OpenRead())
            {
// sample rate
// duration in frames

// IMA4 or
// MS ADPCM: block size (default 512 for mono, 1024 for stereo, interruptible)

// sample count (automatic for IMA4 and PC demo)


// size in raw / offset in raw
                if (sndd.IsMacFile)
                {
                    sound.ChannelCount = (reader.ReadInt32() >> 1) + 1; // TODO: interpret the low bit? (uncompressed/compressed?)
                    sound.SampleRate = 22050;
                    sound.nGameTicks = (UInt16)reader.ReadInt32();

                    sound.IsIMA4 = true;
                    sound.IsPCM = false;
                }
                else
                {
                    sound.IsPCM = false;
                    reader.Skip(4); // flags (1=?, 2=?, 4=?, 8=compressed) TODO: Try uncompressed (even for demo?)
                    reader.Skip(2); // Int16; format ID (2= ADPCM, 1=PCM?) TODO: Try uncompressed (even for demo?)
                    sound.ChannelCount = reader.ReadInt16();
                    sound.SampleRate = reader.ReadInt32();
                    sound.AverageDataRate = reader.ReadInt32(); // in B/s (2 or 4 for PCM; 11155, 22311 or 22179 for Vanilla ADPCM)
                    sound.BlockAlignment = reader.ReadInt16(); // 2 or 4 bytes per block for PCM; 512 or 1024 for Vanilla ADPCM
                    sound.BitsPerSample = reader.ReadInt16(); // bits per sample per channel (4 bits per sample for ADPCM, 16 bits for PCM)
                    sound.HeaderSizeADPCM = reader.ReadInt16(); // size of additional (ADPCM) header (zero for PCM); typically 32 bytes

                    sound.SamplesPerBlock = reader.ReadUInt16(); // UInt16; samples per block (can be inferred from block alignment etc)

                    sound.nCoefPairs = reader.ReadUInt16(); // usually 7
                    sound.CoefADPCMa = new Int16[sound.nCoefPairs]; // usually     256     512     0       192     240     460     392
                    sound.CoefADPCMb = new Int16[sound.nCoefPairs]; // usually     0      -256     0       64      0      -208    -232
                    for (int coefPair = 0; coefPair < sound.nCoefPairs; ++coefPair)
                    {
                        sound.CoefADPCMa[coefPair] = reader.ReadInt16();
                        sound.CoefADPCMb[coefPair] = reader.ReadInt16();
                    }

                    sound.nGameTicks = reader.ReadUInt16(); // UInt16; number of game ticks (truncated to lower value)
                    sound.IsIMA4 = false;
                }

                dataSize = reader.ReadInt32();
                dataOffset = reader.ReadInt32();
            }

            using (var rawReader = sndd.GetRawReader(dataOffset))
                sound.Data = rawReader.ReadBytes(dataSize);

            if (sound.IsIMA4 && do_pc_demo_test) // check if the raw data actually looks like IMA4
            {
                int nIMABlocks = sound.Data.Length / 34;
                int remainder = sound.Data.Length - nIMABlocks * 34;
                if (remainder == 0 && (nIMABlocks % sound.ChannelCount) == 0)
                {
                    bool stepIndexAbove88 = false;
                    for (int ii = 0; ii < nIMABlocks; ii++)
                    {
                        Byte cc = sound.Data[ii * 34 + 1];
                        if ((cc & 0x7F) > 88)
                            stepIndexAbove88 = true;
                    }
                    if (stepIndexAbove88)
                        sound.IsIMA4 = false;
                }
                else
                    sound.IsIMA4 = false;
                if (!(sound.IsIMA4)) // fill in default values of WAV (MS ADCPM) header
                {
                    sound.IsPCM = false;
                    Console.WriteLine("PC-demo MS ADPCM detected; use -nodemo flag to treat as Mac IMA4.");
                    sound.AverageDataRate = (sound.ChannelCount == 1) ? 11155 : 22311;
                    sound.BlockAlignment = (sound.ChannelCount == 1) ? 512 : 1024;
                    sound.BitsPerSample = 4;
                    sound.SamplesPerBlock = 1012;
                    sound.HeaderSizeADPCM = 32;
                    sound.nCoefPairs = 7;
                    sound.CoefADPCMa = new Int16[7]; // usually     256     512     0       192     240     460     392
                    sound.CoefADPCMb = new Int16[7]; // usually     0      -256     0       64      0      -208    -232
                    sound.CoefADPCMa[0] = 256;  sound.CoefADPCMb[0] =    0;
                    sound.CoefADPCMa[1] = 512;  sound.CoefADPCMb[1] = -256;
                    sound.CoefADPCMa[2] =   0;  sound.CoefADPCMb[2] =    0;
                    sound.CoefADPCMa[3] = 192;  sound.CoefADPCMb[3] =   64;
                    sound.CoefADPCMa[4] = 240;  sound.CoefADPCMb[4] =    0;
                    sound.CoefADPCMa[5] = 460;  sound.CoefADPCMb[5] = -208;
                    sound.CoefADPCMa[6] = 392;  sound.CoefADPCMb[6] = -232;
                }
            }
            // validate data and calculate sample count
            if(sound.IsIMA4) // get the sample count
            {
                int nIMABlocks = sound.Data.Length / 34 / sound.ChannelCount;
                if(sound.Data.Length - nIMABlocks * 34 * sound.ChannelCount > 0)
                    throw new InvalidDataException("IMA4 data shouldn't have incomplete blocks.");
                sound.SampleCount = nIMABlocks * 64;
            }
            else
            {
                // TODO: validate all the parameters: resolve conflicts if any, or bail out
                // TODO: handle PCM (the following assumes MS ADPCM)
                if (sound.IsPCM)
                {
                }
                else
                {
                    int wholeBlocks = sound.Data.Length / sound.BlockAlignment;
                    int leftoverBytes = sound.Data.Length - (wholeBlocks * sound.BlockAlignment);
                    int leftoverSamples = 8 * (leftoverBytes - 7 * sound.ChannelCount)
                                          / sound.BitsPerSample / sound.ChannelCount + 2; // assuming 4 bits per sample
                    sound.SampleCount = wholeBlocks * sound.SamplesPerBlock + leftoverSamples;
                }
            }
            return sound;
        }
    }
}