OK, presumably it used to beep / buzz at you and now doesn't?
Let's assume it has one and has stopped squeaking. It either means that the buzzer itself is gubbed, or the signal to make it buzz isn't getitng to it.
Assuming not much changed in the remotes wiring, there will be a wire from the engine to the buzzer (probably Tan or tan with a stripe coloured) If you can break into the loom with 12V across that wire & the earth (an old Hornby train controller or a couple of wires from the 12V supply to the engine) it should squeal at you. If it doesnlt then it;s either the buzzer or one of the connections.
Option 2 is of course to dismantle the remotes & just apply 12V to the buzzer directly, but if you are going to do that, remove the remotes from the side of the console & lay them down flat first! (Don't ask me how I know that helps