fix/ducking again (#79)

when stopping a audio service self.current is set to None, we lose the reference and are unable to restore volume because of that (race condition)

ensure that when we stop a audio service we restore the default volume

ovos_audio/audio.py

@@ -282,6 +282,10 @@ def _perform_stop(self, message=None):
                                   {"by": "audio:" + name})
                 self.bus.emit(msg)
 
+            # ensure we don't leave the volume ducked
+            self.current.restore_volume()
+            self.volume_is_low = False
+
         self.current = None
 
     @require_native_source()