sdl 保存pcm，实现倍速播放声音。

1.解码后 需要进行转换

SwrContext *swrContext = swr_alloc();
            AVSampleFormat dst_format = AV_SAMPLE_FMT_S16;//av_get_packed_sample_fmt((AVSampleFormat)frame->format);
            Uint64 dst_layout = av_get_default_channel_layout(frame->channels);
            // 设置解码参数
            swrContext = swr_alloc_set_opts(nullptr, dst_layout, dst_format, frame->sample_rate,
                frame->channel_layout, (AVSampleFormat)frame->format, frame->sample_rate, 0, nullptr);

            if (!swrContext || swr_init(swrContext) < 0)
                return -1;
            uint8_t *outBuffer;
            av_samples_alloc(&outBuffer, NULL, 2, frame->nb_samples, AV_SAMPLE_FMT_S16, 0);
            int nbSamples = swr_convert(swrContext, &outBuffer, frame->nb_samples, (const uint8_t **)frame->data, frame->nb_samples);

            // 写入PCM数据到文件
            fwrite(outBuffer, 1, nbSamples * 2 * 2, audio_dst_file3); // 2 channels * 2 bytes per sample

            av_freep(&outBuffer);

2.倍速播放，需要将参数的频率改变。

    SDL_AudioSpec desired;
    desired.freq = audio_ctx->sample_rate*1.5; //想实现倍速播放，直接修改频率
    desired.channels = audio_ctx->channels;
    desired.format = AUDIO_S16SYS;
    desired.samples = 1024;
    desired.silence = 0;
    desired.userdata = this;
    desired.callback = audio_callback;

    if (SDL_OpenAudio(&desired, nullptr) < 0)
    {
        return false;
    }

    SDL_PauseAudio(0); // playing