Audio ducking

<div class="wrap">
  <div class="meters">
    <div class="row"><span class="lbl">배경음</span><div class="meter"><i id="bgbar"></i></div></div>
    <div class="row"><span class="lbl">안내음</span><div class="meter"><i id="fgbar"></i></div></div>
  </div>
  <button class="play" id="play" type="button" aria-label="play sound"><span class="tri"></span></button>
</div>

.wrap{position:relative;width:94%;height:80%;display:flex;align-items:center;gap:6%}
.meters{flex:1;display:flex;flex-direction:column;justify-content:center;gap:16%}
.row{display:flex;align-items:center;gap:10px}
.lbl{width:34%;font:600 11px/1 monospace;color:var(--muted);text-align:right}
.meter{flex:1;height:22px;border-radius:11px;background:var(--line);overflow:hidden}
.meter i{display:block;height:100%;border-radius:11px;background:var(--accent);width:30%}
#fgbar{background:var(--accent-2);width:0}
.play{width:clamp(28px,15%,40px);aspect-ratio:1;border-radius:50%;border:1px solid var(--line);background:var(--surface);display:grid;place-items:center;cursor:pointer;box-shadow:0 2px 8px rgba(0,0,0,.12);flex-shrink:0}
@media (min-width:460px){
  .lbl{font-size:15px}
  .meter{height:34px;border-radius:17px}
  .meter i{border-radius:17px}
  .play{width:56px;height:56px}
  .play .tri{border-width:9px 0 9px 13px;margin-left:3px}
}
.play .tri{width:0;height:0;border-style:solid;border-width:6px 0 6px 9px;border-color:transparent transparent transparent var(--fg);margin-left:2px}
.play:active{transform:scale(.92)}

const bg = document.getElementById('bgbar');
const fg = document.getElementById('fgbar');
const CYCLE = 3000;
function idle(t) {
  const p = (t % CYCLE) / CYCLE;
  let bgLevel = 32 + Math.sin(t * 0.004) * 6;
  let fgLevel = 0;
  if (p > 0.4 && p < 0.6) {
    const dip = 1 - Math.abs(p - 0.5) / 0.1;
    bgLevel *= 1 - dip * 0.55;
    fgLevel = dip * 60;
  }
  bg.style.width = bgLevel + '%';
  fg.style.width = fgLevel + '%';
  requestAnimationFrame(idle);
}
requestAnimationFrame(idle);
document.getElementById('play').addEventListener('click', () => {
  const ctx = new (window.AudioContext || window.webkitAudioContext)();
  const now = ctx.currentTime;
  const bgGain = ctx.createGain();
  bgGain.connect(ctx.destination);
  bgGain.gain.setValueAtTime(0.05, now);
  bgGain.gain.linearRampToValueAtTime(0.018, now + 0.5);
  bgGain.gain.linearRampToValueAtTime(0.05, now + 0.9);
  bgGain.gain.linearRampToValueAtTime(0.0001, now + 1.2);
  const bgOsc = ctx.createOscillator();
  bgOsc.type = 'sine';
  bgOsc.frequency.value = 220;
  bgOsc.connect(bgGain);
  bgOsc.start(now);
  bgOsc.stop(now + 1.2);
  const fgGain = ctx.createGain();
  fgGain.connect(ctx.destination);
  fgGain.gain.setValueAtTime(0, now + 0.45);
  fgGain.gain.linearRampToValueAtTime(0.12, now + 0.5);
  fgGain.gain.exponentialRampToValueAtTime(0.0001, now + 0.85);
  const fgOsc = ctx.createOscillator();
  fgOsc.type = 'triangle';
  fgOsc.frequency.value = 660;
  fgOsc.connect(fgGain);
  fgOsc.start(now + 0.45);
  fgOsc.stop(now + 0.9);
  setTimeout(() => ctx.close(), 1400);
});