SDL_mixer Examples
This example code creates a mixer, loads a single sound, and plays it on loop, letting the user seek around in playback with a slider.

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/* seeking.c ... */

/*
 * This example code creates a mixer, loads a single sound, and plays it on
 * loop, letting the user seek around in playback with a slider.
 *
 * This code is public domain. Feel free to use it for any purpose!
 */

#define SDL_MAIN_USE_CALLBACKS 1  /* use the callbacks instead of main() */
#include <SDL3/SDL.h>
#include <SDL3/SDL_main.h>
#include <SDL3_mixer/SDL_mixer.h>

/* We will use this renderer to draw into this window every frame. */
static SDL_Window *window = NULL;
static SDL_Renderer *renderer = NULL;
static MIX_Mixer *mixer = NULL;
static MIX_Audio *audio = NULL;
static MIX_Track *track = NULL;

#define KNOB_WIDTH 30
static const SDL_FRect progressbar = { 120, 225, 400, 75 };

/* This function runs once at startup. */
SDL_AppResult SDL_AppInit(void **appstate, int argc, char *argv[])
{
    char *path = NULL;
    SDL_PropertiesID options = 0;

    SDL_SetAppMetadata("Example Seeking", "1.0", "com.example.seeking");

    /* this doesn't have to run very much, so give up tons of CPU time between iterations. Optional! */
    SDL_SetHint(SDL_HINT_MAIN_CALLBACK_RATE, "5");

    /* we don't need video, but we'll make a window for smooth operation. */
    if (!SDL_Init(SDL_INIT_VIDEO)) {
        SDL_Log("Couldn't initialize SDL: %s", SDL_GetError());
        return SDL_APP_FAILURE;
    }

    if (!SDL_CreateWindowAndRenderer("examples/basic/seeking", 640, 480, SDL_WINDOW_RESIZABLE, &window, &renderer)) {
        SDL_Log("Couldn't create window/renderer: %s", SDL_GetError());
        return SDL_APP_FAILURE;
    }

    if (!MIX_Init()) {
        SDL_Log("Couldn't init SDL_mixer library: %s", SDL_GetError());
        return SDL_APP_FAILURE;
    }

    /* Create a mixer on the default audio device. Don't care about the specific audio format. */
    mixer = MIX_CreateMixerDevice(SDL_AUDIO_DEVICE_DEFAULT_PLAYBACK, NULL);
    if (!mixer) {
        SDL_Log("Couldn't create mixer on default device: %s", SDL_GetError());
        return SDL_APP_FAILURE;
    }

    /* load a sound file */
    SDL_asprintf(&path, "%smusic.mp3", SDL_GetBasePath());  /* allocate a string of the full file path */
    audio = MIX_LoadAudio(mixer, path, false);
    if (!audio) {
        SDL_Log("Couldn't load %s: %s", path, SDL_GetError());
        SDL_free(path);
        return SDL_APP_FAILURE;
    }

    SDL_free(path);  /* done with this, the file is loaded. */

    /* we need a track on the mixer to play the audio. Each track has audio assigned to it, and
       all playing tracks are mixed together for the final output. */

    track = MIX_CreateTrack(mixer);
    if (!track) {
        SDL_Log("Couldn't create a mixer track: %s", SDL_GetError());
        return SDL_APP_FAILURE;
    }
    MIX_SetTrackAudio(track, audio);

    /* start the audio playing! */
    options = SDL_CreateProperties();
    if (!options) {
        SDL_Log("Couldn't create play options: %s", SDL_GetError());
        return SDL_APP_FAILURE;
    }

    /* note these all use MILLISECONDS, since the track is generic, but you can use equivalent FRAMES properties, for frame-perfect mixing. */
    SDL_SetNumberProperty(options, MIX_PROP_PLAY_LOOPS_NUMBER, -1);  /* loop forever */
    MIX_PlayTrack(track, options);
    SDL_DestroyProperties(options);  /* MIX_PlayTrack makes a copy of the options, so this can go away. */

    return SDL_APP_CONTINUE;  /* carry on with the program! */
}

static void seek_audio(float x, float y)
{
    const SDL_FPoint pt = { x, y };
    if (SDL_PointInRectFloat(&pt, &progressbar)) {
        /* seek to a new position in the track, based on where the mouse clicked/dragged in the progress bar. */
        const float pct = (pt.x - progressbar.x) / progressbar.w;
        MIX_SetTrackPlaybackPosition(track, (Sint64) ((float) MIX_GetAudioDuration(audio) * pct));
    }
}

/* This function runs when a new event (mouse input, keypresses, etc) occurs. */
SDL_AppResult SDL_AppEvent(void *appstate, SDL_Event *event)
{
    if (event->type == SDL_EVENT_QUIT) {
        return SDL_APP_SUCCESS;  /* end the program, reporting success to the OS. */
    } else if (event->type == SDL_EVENT_MOUSE_BUTTON_DOWN) {
        if (event->button.button == 1) {
            seek_audio(event->button.x, event->button.y);
        }
    } else if (event->type == SDL_EVENT_MOUSE_MOTION) {
        if (event->motion.state & SDL_BUTTON_LMASK) {
            seek_audio(event->motion.x, event->motion.y);
        }
    }
    return SDL_APP_CONTINUE;  /* carry on with the program! */
}

/* This function runs once per frame, and is the heart of the program. */
SDL_AppResult SDL_AppIterate(void *appstate)
{
    SDL_FRect knob = { progressbar.x, progressbar.y, KNOB_WIDTH, progressbar.h };
    knob.x += (progressbar.w - knob.w) * (((float) MIX_GetTrackPlaybackPosition(track)) / ((float) MIX_GetAudioDuration(audio)));

    SDL_SetRenderDrawColor(renderer, 0, 0, 100, 255);
    SDL_RenderClear(renderer);
    SDL_SetRenderDrawColor(renderer, 255, 255, 0, 255);
    SDL_RenderFillRect(renderer, &progressbar);
    SDL_SetRenderDrawColor(renderer, 255, 0, 0, 255);
    SDL_RenderFillRect(renderer, &knob);

    SDL_RenderPresent(renderer);

    return SDL_APP_CONTINUE;  /* carry on with the program! */
}

/* This function runs once at shutdown. */
void SDL_AppQuit(void *appstate, SDL_AppResult result)
{
    /* SDL will clean up the window/renderer for us, MIX_Quit() destroys any mixer objects we made. */
    MIX_Quit();
}