Mike,
You mentioned that the engine ran well, l'm not certain if you are referring to how it operates in running around on the track, or just the way sounds are perfoming.
You might pull the tender shell and take a look at the speaker, it might be that the speaker or the speaker enclosure has failed. I would look inside the tender before assuming the board has gone bad. If you have another 8 ohm speaker you could try connecting it but keep in mind the enclosure for the speaker is very important.
Ray