I don't know if I should be concerned about your comment saying the loco didn't respond to the restore procedure. I can understand the sound system not responding, as that's what we're trying to get working, but you should have seen a flickering headlight to indicate the loco was responding. Can you change the loco's ID # (NOT the same as the reset code) without a problem?
Now back to the original hypothesis that your tender's sound system is not receiving the serial commands from the locomotive. One of three things could be the cause: the LCRU2 board in the loco isn't sending the commands, the tether isn't carrying them to the tender, or the RS2.5 board in the tender is unable to receive the commands. Unfortunately, the LCRU2 board and RS2.5 board aren't that simple to test without having exact identical units to swap out, but you can check for continuity of the tether.
I've attached the wiring diagrams for both the loco and tender. I've marked the only tether wire that's involved - you've already proven the other ones are OK. Check for continuity of the brown tether wire in the locomotive to the purple tether wire in the tender, preferably at the points where they terminate into the boards.
If that tether wire looks good, then it's one of the boards at either end. A tech could troubleshoot the two boards to determine which is the problem, but there isn't an easy way to walk you through that part.
I hope this helps.
P.S. Note the tender wiring diagram shown below is wrong for hooking up the speaker. The open pin of that connector should be in the center. Your loco is likely wired correctly if you have any sound. I just didn't want you to go down a wrong path.