Which engine is it you need help with? There are at least 5 variations of the polar express and a 6th tender variation that I am aware of. Lionel's 6-## part numbers would be helpful. If it is a LionChief model, then the tender from another LionChief engine should work for testing, however all that is in that tender is a speaker, all of the electronics are in the engine on one board. Also on the LionChief models, all three I am aware of, the wires connecting from the board to the drawbar and on to the speaker are very tiny, and one could have broken. Something to check for.
As for the conventional models, I'm unsure what the sound systems are and if other tenders would work with them.
JGL