+ *
+ * Note: this module supports two separate sound mechanisms concurrently.
+ *
+ * 1. This module will load and play a set of sound effects defined in an
+ * xml file and tie them to various property states.
+ * 2. This modules also maintains a queue of 'message' audio files. These
+ * are played sequentially with no overlap until the queue is finished.
+ * This second mechanims is useful for things like tutorial messages or
+ * background atc chatter.