cat sdl_audio.cpp
#include <SDL2/SDL.h>

#define MUS_PATH "test.wav"

// prototype for our audio callback
// see the implementation for more information
void my_audio_callback(void *userdata, Uint8 *stream, int len);

// variable declarations
static Uint8 *audio_pos; // global pointer to the audio buffer to be played
static Uint32 audio_len; // remaining length of the sample we have to play


/*
** PLAYING A SOUND IS MUCH MORE COMPLICATED THAN IT SHOULD BE
*/
int main(int argc, char* argv[]){

        // Initialize SDL.
        if (SDL_Init(SDL_INIT_AUDIO) < 0)
                        return 1;

        // local variables
        static Uint32 wav_length; // length of our sample
        static Uint8 *wav_buffer; // buffer containing our audio file
        static SDL_AudioSpec wav_spec; // the specs of our piece of music


        /* Load the WAV */
        // the specs, length and buffer of our wav are filled
        if( SDL_LoadWAV(MUS_PATH, &wav_spec, &wav_buffer, &wav_length) == NULL ){
          return 1;
        }
        // set the callback function
        wav_spec.callback = my_audio_callback;
        wav_spec.userdata = NULL;
        // set our global static variables
        audio_pos = wav_buffer; // copy sound buffer
        audio_len = wav_length; // copy file length

        /* Open the audio device */
        if ( SDL_OpenAudio(&wav_spec, NULL) < 0 ){
          fprintf(stderr, "Couldn't open audio: %s\n", SDL_GetError());
          exit(-1);
        }

        /* Start playing */
        SDL_PauseAudio(0);

        // wait until we're don't playing
        while ( audio_len > 0 ) {
                SDL_Delay(100);
        }

        // shut everything down
        SDL_CloseAudio();
        SDL_FreeWAV(wav_buffer);

}

// audio callback function
// here you have to copy the data of your audio buffer into the
// requesting audio buffer (stream)
// you should only copy as much as the requested length (len)
void my_audio_callback(void *userdata, Uint8 *stream, int len) {

        if (audio_len ==0)
                return;

        len = ( len > audio_len ? audio_len : len );
        //SDL_memcpy (stream, audio_pos, len);                                  // simply copy from one buffer into the other
        SDL_MixAudio(stream, audio_pos, len, SDL_MIX_MAXVOLUME);// mix from one buffer into another

        audio_pos += len;
        audio_len -= len;
}