Purchased a Tweetsie set last Christmas and  am now having a problem with the sound. When the train is first started the sound works as it should but over the next few minutes the volume of the sound decreases until there is no sound at all. Have narrowed the problem to the sound board in the tender. A good 9v battery is installed. The sound worked fine until about a month ago. Any suggestions appreciated!! This happens every time the train is run.

Doc Halstead