Final response, I'm impressed! Free evaluation.
You might want to first check the connector between the loco and tender. With power on the track, move the connector around a bit and see if there is any operation. If it operates sporadically, that points to a bad wire inside the connector. If you get nothing at all, that points to something else besides the connector.
You are welcome to pack the tender up with plenty of bubble wrap and ship just the tender to us. There is no charge to check it or to do a reset back tot he factory defaults, which hopefully would fix it. If it does indeed need a new main sound decoder board, the charge would be about $100.
You can address it to the repair department at the address below. Be sure to include your complete contact info and a description of the issue in the package.