sound = playSound("sound.mp3")

function Audio()
    local vol=getSoundVolume(sound)
    vol=vol-0.1
    if (vol<0) then
    stopSound(sound)
    return
    end
    setSoundVolume(sound,vol)
    setTimer(Audio, 300, 1)
end

local sound = playSound('sound.mp3');
local vol; local startTime;

local function soundFadeRender()
    local elapsedTime = getTickCount() - startTime;
    local progress = elapsedTime / 3000;

    _, _, vol = interpolateBetween(
        0, 0, 1,
        0, 0, 0,
        progress, 'InOutQuad');

    setSoundVolume(sound, vol);

    if vol <= 0 then
        stopSound(sound);
        removeEventHandler('onClientRender', root, soundFadeRender);
    end
end

local function startSoundFade()
    assert(not isSoundPaused(sound), 'sound is paused');
    startTime = getTickCount();
    addEventHandler('onClientRender', root, soundFadeRender);
end