Fix playing multiple sounds at once

LeCountdown/Conductor.swift

@@ -313,6 +313,7 @@ class Conductor: ObservableObject {
     }
     
     fileprivate func _playSound(_ filename: String, duration: TimeInterval? = nil) {
+        self.soundPlayer?.stop()
         do {
             let soundFile = try SoundFile(fullName: filename)
             let soundPlayer = SoundPlayer()