参考资料：

FFmpeg和SDL2播放mp4_sdl 播放mp4 声音-CSDN博客

SimplePlayer/SimplePlayer.c at master · David1840/SimplePlayer · GitHub

在前面的学习中，通过获得的AVFrame进行了播放画面，

[FFmpeg学习]初级的SDL播放mp4测试-CSDN博客

播放音频原理类似，也是获取AVFrame的信息，

extern "C" {
#include <libavutil/log.h>
#include <libavformat/avformat.h>
#include <libavcodec/avcodec.h>
#include <libavformat/avformat.h>
#include <libswresample/swresample.h>
}
// Simplest FFmpeg Sync Player.cpp : 定义控制台应用程序的入口点。
//

#include <stdio.h>
#include <SDL_types.h>
#include "SDL.h"


static Uint8* audio_chunk;
static Uint32 audio_len;
static Uint8* audio_pos;

#define MAX_AUDIO_FRAME_SIZE 19200


//音频设备需要更多数据的时候会调用该回调函数
void read_audio_data(void* udata, Uint8* stream, int len) {
    fprintf(stderr, "stream addr:%p, audio_len:%d, len:%d\n",
        stream,
        audio_len,
        len);
    //首先使用SDL_memset()将stream中的数据设置为0
    SDL_memset(stream, 0, len);
    if (audio_len == 0)
        return;
    len = (len > audio_len ? audio_len : len);

    SDL_MixAudio(stream, audio_pos, len, SDL_MIX_MAXVOLUME);
    audio_pos += len;
    audio_len -= len;
}

#undef main
int main(int argc, char* argv[]) {
    const char* file = "test.mp4";

    AVFormatContext* pFormatCtx = NULL; //for opening multi-media file

    int i, audioStream = -1;

    AVCodecParameters* pCodecParameters = NULL; //codec context
    AVCodecContext* pCodecCtx = NULL;

    const AVCodec* pCodec = NULL; // the codecer
    AVFrame* pFrame = NULL;
    AVPacket* packet;
    uint8_t* out_buffer;

    int64_t in_channel_layout;
    struct SwrContext* au_convert_ctx;

    if (avformat_open_input(&pFormatCtx, file, NULL, NULL) != 0) {
        SDL_LogError(SDL_LOG_CATEGORY_APPLICATION, "Failed to open video file!");
        return -1; // Couldn't open file
    }

    audioStream = av_find_best_stream(pFormatCtx, AVMEDIA_TYPE_AUDIO, -1, -1, NULL, 0);

    if (audioStream == -1) {
        SDL_LogError(SDL_LOG_CATEGORY_APPLICATION, "Din't find a video stream!");
        return -1;// Didn't find a video stream
    }

    // Get a pointer to the codec context for the video stream
    pCodecParameters = pFormatCtx->streams[audioStream]->codecpar;

    // Find the decoder for the video stream
    pCodec = avcodec_find_decoder(pCodecParameters->codec_id);
    if (pCodec == NULL) {
        SDL_LogError(SDL_LOG_CATEGORY_APPLICATION, "Unsupported codec!\n");
        return -1; // Codec not found
    }

    // Copy context
    pCodecCtx = avcodec_alloc_context3(pCodec);
    if (avcodec_parameters_to_context(pCodecCtx, pCodecParameters) != 0) {
        SDL_LogError(SDL_LOG_CATEGORY_APPLICATION, "Couldn't copy codec context");
        return -1;// Error copying codec context
    }

    // Open codec
    if (avcodec_open2(pCodecCtx, pCodec, NULL) < 0) {
        SDL_LogError(SDL_LOG_CATEGORY_APPLICATION, "Failed to open decoder!\n");
        return -1; // Could not open codec
    }
    packet = (AVPacket*)av_malloc(sizeof(AVPacket));
    av_init_packet(packet);
    pFrame = av_frame_alloc();

    uint64_t out_channel_layout = AV_CH_LAYOUT_STEREO;//输出声道
    int out_nb_samples = 1024;
    enum AVSampleFormat out_sample_fmt = AV_SAMPLE_FMT_S16;//输出格式S16
    int out_sample_rate = 44100;
    int out_channels = av_get_channel_layout_nb_channels(out_channel_layout);

    int out_buffer_size = av_samples_get_buffer_size(NULL, out_channels, out_nb_samples, out_sample_fmt, 1);
    out_buffer = (uint8_t*)av_malloc(MAX_AUDIO_FRAME_SIZE * 2);


    //Init
    if (SDL_Init(SDL_INIT_AUDIO | SDL_INIT_TIMER)) {
        printf("Could not initialize SDL - %s\n", SDL_GetError());
        return -1;
    }

    SDL_AudioSpec spec;
    spec.freq = out_sample_rate;
    spec.format = AUDIO_S16SYS;
    spec.channels = out_channels;
    spec.silence = 0;
    spec.samples = out_nb_samples;
    spec.callback = read_audio_data;
    spec.userdata = pCodecCtx;

    if (SDL_OpenAudio(&spec, NULL) < 0) {
        printf("can't open audio.\n");
        return -1;
    }

    in_channel_layout = av_get_default_channel_layout(pCodecCtx->channels);
    printf("in_channel_layout --->%d\n", in_channel_layout);
    au_convert_ctx = swr_alloc();
    au_convert_ctx = swr_alloc_set_opts(au_convert_ctx, out_channel_layout, out_sample_fmt, out_sample_rate,
        in_channel_layout, pCodecCtx->sample_fmt, pCodecCtx->sample_rate, 0, NULL);
    swr_init(au_convert_ctx);

    SDL_PauseAudio(0);

    while (av_read_frame(pFormatCtx, packet) >= 0) {
        if (packet->stream_index == audioStream) {
            avcodec_send_packet(pCodecCtx, packet);
            while (avcodec_receive_frame(pCodecCtx, pFrame) == 0) {
                swr_convert(au_convert_ctx, &out_buffer, MAX_AUDIO_FRAME_SIZE, (const uint8_t**)pFrame->data,
                    pFrame->nb_samples); // 转换音频
            }

            audio_chunk = (Uint8*)out_buffer;
            audio_len = out_buffer_size;
            audio_pos = audio_chunk;

            while (audio_len > 0) {
                SDL_Delay(1);//延迟播放
            }
        }
        av_packet_unref(packet);
    }
    swr_free(&au_convert_ctx);
    SDL_Quit();

    return 0;
}

在windows上，如果有方法废弃的错误，

错误    C4996    'AVCodecContext::channels': 被声明为已否决    

可以设置SDL检查为否，来解决

代码流程里，需要注意一下audio_callback是怎么回调的，

audio_callback函数是由SDL音频系统在需要更多音频数据以填充音频缓冲区时自动调用的。当SDL音频系统开始播放音频时，它会周期性地调用音频回调函数以获取新的音频数据。音频回调函数的调用频率取决于音频采样率和缓冲区大小。

在main函数中，我们初始化SDL音频系统并设置了音频回调函数audio_callback：

if (SDL_Init(SDL_INIT_AUDIO) < 0) {
    fprintf(stderr, "Could not initialize SDL - %s\n", SDL_GetError());
    exit(1);
}

// 设置音频回调函数
SDL_AudioSpec desired;
desired.freq = audioCodecContext->sample_rate;
desired.format = AUDIO_S16SYS; // 使用16位有符号小端字节序样本
desired.channels = audioCodecContext->channels;
desired.samples = 4096; // 音频缓冲区大小，可以根据需要调整
desired.callback = audio_callback;
desired.userdata = NULL;

// 打开音频设备
SDL_AudioDeviceID deviceId = SDL_OpenAudioDevice(NULL, 0, &desired, NULL, SDL_AUDIO_ALLOW_ANY_CHANGE);
if (deviceId <= 0) {
    fprintf(stderr, "Failed to open audio device: %s\n", SDL_GetError());
    return -1;
}

// 开始播放音频
SDL_PauseAudioDevice(deviceId, 0);

当我们调用SDL_OpenAudioDevice函数时，SDL音频系统会注册音频回调函数audio_callback。当我们调用SDL_PauseAudioDevice(deviceId, 0)开始播放音频时，SDL音频系统会根据音频参数（如采样率、通道数和缓冲区大小）定期调用audio_callback函数。

在audio_callback函数中，我们需要根据音频缓冲区的需求提供音频数据。这通常涉及从文件或实时流中解码音频数据，并将其传递给SDL音频系统。在我们的示例中，我们从全局变量audio_buffer中读取音频数据，该变量由swr_convert函数填充。

总之，audio_callback函数是由SDL音频系统在需要更多音频数据以填充音频缓冲区时自动触发的。您无需手动调用此函数，SDL音频系统会负责调用它。只需确保在回调函数中提供正确的音频数据即可。