声量远近：音效随距离的衰减分级

-- 错误：全图同音量，远处的脚步照响
function playAt(id)
    audioPlay(id, 1)
end

local function playByDist(id, dist)
    if dist > 1200 then return end
    local vol = 1
    if dist > 600 then
        vol = 0.4
    elseif dist > 300 then
        vol = 0.7
    end
    audioPlay(id, vol)
end