There might be a good way to morph one sound into another. I just don't know it. But fading one sound in while fading another out works quite well.
var min_vol: float = 0.1 var max_vol: float = 0.6 # $Player.chest_to_world() returns a float from 0.0 to 1.0 func _process(_delta: float) -> void: $BreatheOut.volume_db = linear_to_db(min_vol+($Player.chest_to_world()*(max_vol-min_vol))) $BreatheIn.volume_db = linear_to_db(max_vol-($Player.chest_to_world()*(max_vol-min_vol)))