AudioParam: setValueAtTime() メソッド

setValueAtTime(value, startTime)

// 音声コンテキストの作成
const AudioContext = window.AudioContext || window.webkitAudioContext;
const audioCtx = new AudioContext();
// 例のための基本的な値を設定
const myAudio = document.querySelector("audio");
const pre = document.querySelector("pre");
const myScript = document.querySelector("script");
pre.innerHTML = myScript.innerHTML;
const targetAtTimePlus = document.querySelector(".set-target-at-time-plus");
const targetAtTimeMinus = document.querySelector(".set-target-at-time-minus");
// MediaElementAudioSourceNode を作成
// Feed the HTMLMediaElement into it
const source = audioCtx.createMediaElementSource(myAudio);
// ゲインノードを作成し、ゲイン値を 0.5 に設定
const gainNode = audioCtx.createGain();
gainNode.gain.value = 0.5;
let currGain = gainNode.gain.value;
// AudioBufferSourceNode を gainNodeに、
// gainNode を出力先に接続
source.connect(gainNode);
gainNode.connect(audioCtx.destination);
// onclick で何かするようにボタンを設定
targetAtTimePlus.onclick = () => {
  currGain += 0.25;
  gainNode.gain.setValueAtTime(currGain, audioCtx.currentTime + 1);
};
targetAtTimeMinus.onclick = () => {
  currGain -= 0.25;
  gainNode.gain.setValueAtTime(currGain, audioCtx.currentTime + 1);
};