If you tried reprogramming with no effect, usually the next step is to open it up and reseat the boards and look for pinched wires. The boards other than that particular audio board are readily available. No need to replace them all with a rectifier.
Pete,
This loco is pre-TMCC and pre-modular RailSounds. You can't reprogram or re-seat boards on these.
Unfortunately, I'm fairly certain the boards in these are unique, and weren't used elsewhere. Unfortunately, a quick online search didn't find anyone with the boards in stock.
My only advice would be to check all connections going to the board, as well as check the functionality of the speaker.
TRW