I did see another post with the same problem but this is a Pro-78 although it may have the same speaker.
I have used the Pro-78 for 3-4 years and could not be happier, it has performed flawlessly all this time.
A month ago the speaker started to sound rough and now the speaker will sometimes work at first but it dies out and remain silent.
I did take it apart to check continuity of the speaker and it is ok.
I was wondering if Michael B has a source where I can purchase a new speaker assuming that is in fact the problem.
A web search does not reveal any source for that particular speaker.