创建一个位置相关的音频对象.
			使用了[link:https://developer.mozilla.org/en-US/docs/Web/API/Web_Audio_API Web Audio API].
		
			[example:webaudio_orientation webaudio / orientation ]
			[example:webaudio_sandbox webaudio / sandbox ]
			[example:webaudio_timing webaudio / timing ]
		
		// create an AudioListener and add it to the camera
		var listener = new THREE.AudioListener();
		camera.add( listener );
		// create the PositionalAudio object (passing in the listener)
		var sound = new THREE.PositionalAudio( listener );
		// load a sound and set it as the PositionalAudio object's buffer
		var audioLoader = new THREE.AudioLoader();
		audioLoader.load( 'sounds/song.ogg', function( buffer ) {
			sound.setBuffer( buffer );
			sound.setRefDistance( 20 );
			sound.play();
		});
		// create an object for the sound to play from
		var sphere = new THREE.SphereGeometry( 20, 32, 16 );
		var material = new THREE.MeshPhongMaterial( { color: 0xff2200 } );
		var mesh = new THREE.Mesh( sphere, material );
		scene.add( mesh );
		// finally add the sound to the mesh
		mesh.add( sound );
		
		listener — (必须) [page:AudioListener AudioListener] 实例.
[page:Audio Audio]类的继承属性.
位置相关音频的[link:https://developer.mozilla.org/en-US/docs/Web/API/PannerNode PannerNode].
[page:Audio Audio]类的继承方法.
返回[page:PositionalAudio.panner panner].
返回[link:https://developer.mozilla.org/en-US/docs/Web/API/PannerNode/refDistance panner.refDistance]的值.
设置[link:https://developer.mozilla.org/en-US/docs/Web/API/PannerNode/refDistance panner.refDistance]的值.
返回[link:https://developer.mozilla.org/en-US/docs/Web/API/PannerNode/rolloffFactor panner.rolloffFactor]的值.
设置[link:https://developer.mozilla.org/en-US/docs/Web/API/PannerNode/rolloffFactor panner.rolloffFactor]的值.
返回[link:https://developer.mozilla.org/en-US/docs/Web/API/PannerNode/distanceModel panner.distanceModel]的值.
设置[link:https://developer.mozilla.org/en-US/docs/Web/API/PannerNode/distanceModel panner.distanceModel]的值.
返回[link:https://developer.mozilla.org/en-US/docs/Web/API/PannerNode/maxDistance panner.maxDistance]的值.
设置[link:https://developer.mozilla.org/en-US/docs/Web/API/PannerNode/maxDistance panner.maxDistance]的值.
这个方法用来把环绕声音转换为定向声音[link:https://developer.mozilla.org/en-US/docs/Web/API/PannerNode directional sound].
[link:https://github.com/mrdoob/three.js/blob/master/src/[path].js src/[path].js]