make Music a singleton script instead of a scene

scripts/audio/music.gd

@@ -2,8 +2,8 @@ extends AudioStreamPlayer
 
 signal track_started
 
-var bpm: float
-var bar_beats: float
+var bpm: float = 100
+var bar_beats: float = 4
 var beat_length: float
 var bar_length: float
 var bpm_factor: float
@@ -13,9 +13,19 @@ var audio_delta: float
 var track_time: float
 var bar_time: float
 
+var default_stream := preload("res://assets/music/test_track.mp3")
+
+
+func _init() -> void:
+	process_mode = PROCESS_MODE_ALWAYS
+	process_priority = -100
+	process_physics_priority = -100
+
 
 func _ready() -> void:
+	stream = default_stream
 	_play_track()
+	bus = "Music"
 
 
 func _process(_delta: float) -> void: