Speakers are a common culprit in sound problems on older sound-equipped Lionel and MTH engines. I have several units in the 8+ year age range with speakers that have deteriorated and need to be replaced. Fortunately, replacing speakers is not a difficult repair on many engines. It's mainly a matter of finding a good source of replacements that don't cost an arm and a leg. Here's a post on the OGR forum outlining how I am approaching the problem with several MTH engines.