用此函数来初始化结构SDL_AudioCVT，为转换作准备。
int SDL_BuildAudioCVT(SDL_AudioCVT* cvt,
SDL_AudioFormat src_format,
Uint8 src_channels,
int src_rate,
SDL_AudioFormat dst_format,
Uint8 dst_channels,
int dst_rate)例子：
void play_sound(const char *file)
{
int index;
SDL_AudioSpec wave;
Uint8 *data;
Uint32 dlen;
SDL_AudioCVT cvt;
/* 寻找一个空的（或者完成了的）音频口 */
for ( index=0; index<NUM_SOUNDS; ++index ) {
if ( sounds[index].dpos == sounds[index].dlen ) {
break;
}
}
if ( index == NUM_SOUNDS )
return;
/* 加载声音文件，并转换成16位、立体声、22kHz格式 */
if ( SDL_LoadWAV(file, &wave, &data, &dlen) == NULL ) {
fprintf(stderr, "无法加载 %s: %s\n", file, SDL_GetError());
return;
}
SDL_BuildAudioCVT(&cvt, wave.format, wave.channels, wave.freq,
AUDIO_S16, 2, 22050);
cvt.buf = (unsigned char *)malloc(dlen*cvt.len_mult);
memcpy(cvt.buf, data, dlen);
cvt.len = dlen;
SDL_ConvertAudio(&cvt);
SDL_FreeWAV(data);
/* 将音频数据放入音频口（立刻开始回放了） */
if ( sounds[index].data ) {
free(sounds[index].data);
}
SDL_LockAudio();
sounds[index].data = cvt.buf;
sounds[index].dlen = cvt.len_cvt;
sounds[index].dpos = 0;
SDL_UnlockAudio();
}