o Fixed several problems related to new standard API for AudioContext
  (see https://developer.mozilla.org/en-US/docs/Web/API/Web_Audio_API/Porting_webkitAudioContext_code_to_standards_based_AudioContext)
  - crateGainNode() -> createGain()
  - AudioBufferSourcenode no longer has built in gain.value (use separate gain
    node instead)
  - AudioBufferSourceNode no longer has playbackState (use onended handler
    handler instead)