Whenever I play music using XMLHttpRequest I must wait at least 10 seconds or so until I hear the sound.
How big is the audio file in question? If it's large, you're probably paying a big price for
createMediaElementSource(), you get to decode the audio file in chunks. It'll start playing back as soon as it thinks it has enough of a buffer to play through to the end.
decodeAudioData, you have to wait until the whole file gets decoded.