Very odd. Your picture of the Lenz Gold Maxi shows that something is plugged into the SUSI interface. It is the small connector at the bottom right of the decoder as per your picture. Its got four wires blue, red, grey, black. Where do they go to if not to the SUSI sound module? Power and data come over that interface so there is no need for any external power for the SUSI module. Just a thought is the speaker in the tender and perhaps the two wires you disconnected are the connection to it?
F7 is not by default set up to anything on the Lenz Gold Maxi (F3 is shunting speed and F4 is momentum off by default). F7 might be something configured in the SUSI module. Do you get individual sounds such as bell, whistle, sound on/off, fader etc on any of the functions?
Sorry I can't help much. Could you contact the previous owner?