John Cullen,
Turn the transformer power off. When turning it back on, see if the sound returns to the engine. It should return. If not, try to re-program the engine. I have had steam engines lose sound after repeated interruptions over my Atlas O switches. After several interruptions going over switches, the system shuts down.
The battery should help keep power to the sound system. Lionel may not have put a 9-volt battery cable in the engine tender. If they did, try a 9-volt battery and see if that keeps the sound on when going over switch gaps.
Gun Runner John's rechargeable battery should help with the sound system.
Sincerely, John Rowlen