Gene, you are not the first user who posted about this issue. I have read 5 to 10 similar postings in the past 2 to 3 years. It can easily happen that a user mutes Streets & Trips, either purposely or accidentally, and then at a later time the user doesn't know why there is no sound.
A user may exit the program, and a few days (or weeks) later, when the program is again used for navigation, there is no sound coming from Microsoft Streets & Trips. A glance at the screen will not reveal that the program has been muted, either.
I think if sound could be enabled (un-muted) each time the program starts, this type of confusion would be prevented.