On the engine, the board on top with the 24 pins is your R4LC board. In the tender, your board on the right is your Railsounds power board and the board on the left is your Railsounds audio board. Do you have a Lionel tech near you? If so he can put your boards into the board tester he has. If not, you can send me the boards and I can test them for you.
Your communication issue is in the engine. The fact that it is not in Legacy remote mode is your issue. Find the wire that connects to your handrails and do a continuity test between the wire and pin 23 on the R4LC board. Looking at the picture of the R4LC, pin 23 (antenna) is the top pin on the left.
Follow the wire that connects to the handrails (usually a grey wire) to a 2 pin plug on the motherboard. When you find the plug, do a continuity test between the pins on the motherboard and pin 23 on your R4LC. If you have no continuity there, you may have a bad trace on your motherboard. If that is the case, let us know and we will give you the shortcut to use instead of buying a motherboard.
Dave, your tender came alive because your engine was still in conventional mode. I have run into these that will drive you crazy. One last thing, pull the plugs off the motherboard one at a time and plug back in.
Problem is antenna not talking to the R4LC or a bad R4LC like the guys stated. Do not go crazy over the tender as your locomotive will operate without it. Concentrate on the things to look for in the engine. The fact that sound is coming up on your tender just tells you that your engine is not in command mode.