Great article on the Class 31, I'm going to do some of these mods on my one. I thought I'd post some details on putting a stay alive and better speaker in it, as I couldn't find any other posts about this online.
The positive connection to the decoder is easy (Blue wire) but the negative is at the end of one of the small diodes and very hard to solder to without damaging the board. Luckily I traced it back to a better/larger point on the other side of the board. With this connected the loco runs over complicated point work without staling now.
I've been using this brand of speakers in my locos for a while. They are the same as used bluetooth speakers and very bassy. They don't work for all locos though for instance my HST didn't sound at all right but in my Class 31's they are great. You have to make sure all the wires are stuck down though as it can rattle its that good.
Speaker link: https://www.parts-express.com/tang-band-t0-2008s-speaker-module-2-5-32-x-1--264-943
Next I have to modify the wheels as with my code 83 Walthers track the flanges are too deep and its bouncing of the sleeps and frogs ! Might give these ago https://www.ultrascale.uk/eshop/products/view/CAT007/414