Fix playing a Sound once as `stereo()` changing the `panningModel` for all future `pos()`itional playback

[Jimbly]

Prior to this change:

sound.pos(0,0,0, sound.play()) // plays with panningModel=HRTF
sound.stero(0, sound.play()) // plays with stereo panner on new browsers, fallback for old
sound.pos(0,0,0, sound.play()) // bug: plays with panningModel=equalpower

After the fix, any future positional playback will use the originally specified/default panningModel as expected.