Reuse Single AudioContext

Reuse a single AudioContext instance; do not create new ones per sound.

Incorrect (new context per call):

function playSound() {
  const ctx = new AudioContext();
}

Correct (singleton):

let audioContext: AudioContext | null = null;

function getAudioContext(): AudioContext {
  if (!audioContext) {
    audioContext = new AudioContext();
  }
  return audioContext;
}