Getting started with audio

The best filters to take a look at if you are searching for a way to get started with an audio filter is the internal audio filters of AviSynth. Mainly audio.cpp is interesting.

Basically you override GetAudio(...) instead of GetFrame, and fill the buffer with data. A simple filter could look like this:

Filter creation - skip if no audio:

AVSValue __cdecl HalfVolume::Create(AVSValue args, void*, IScriptEnvironment* env) {
  if (!args[0].AsClip()->GetVideoInfo().AudioChannels())
    return args[0];

  return new HalfVolume(args[0].AsClip());
}

Constructor:

HalfVolume::HalfVolume(PClip _child)
    : GenericVideoFilter(ConvertAudio::Create(_child, SAMPLE_INT16 | SAMPLE_FLOAT, SAMPLE_FLOAT)) {   
}

This is a bit tricky. It'll require you to include ConvertAudio.cpp. What it does is automatic sample type conversion. Basically what it does is that you tell that your filter supports SAMPLE_INT16 and SAMPLE_FLOAT, and that it prefers SAMPLE_FLOAT. If the input isn't 16 bit or float, it'll be converted to float.

GetAudio override:

void __stdcall HalfVolume::GetAudio(void* buf, __int64 start, __int64 count, IScriptEnvironment* env) {
  child->GetAudio(buf, start, count, env);
  int channels = vi.AudioChannels();

  if (vi.SampleType() == SAMPLE_INT16) {
    short* samples = (short*)buf;
    for (int i=0; i< count; i++) {
      for(int j=0;j< channels;j++) {
         samples[i*channels+j] /= 2;
      }
    }
  } else if (vi.SampleType() == SAMPLE_FLOAT) {
    SFLOAT* samples = (SFLOAT*)buf;
    for (int i=0; i< count; i++) {
      for(int j=0;j< channels;j++) {
         samples[i*channels+j] /= 2.0f;
      }
    }
  }
}

Implementation of a half volume filter. Very explicit, so it isn't going to be the fastest possible, but it should serve the purpose. Furthermore have a look discussion here and look also at audio.cpp for a bunch of more advanced stuff. A lot of technical details are also to be found in AviSynth Two-Five Audio.