For various reasons, I'm in the process of ruining a Hornby Class 40 fitted with TTS sound.
It's the first sound fitted loco I've ever owned and running the model on the club OO layout, I was pretty impressed. OK, I can't remember what a Peak sounds like but to me it was making diesel noises and I played with it like a new toy blowing the horn to anoy the nearby P4 crowd.
Anyway, body removed, here's what's inside. A chip, that appears to be a standard fitting movable to other locos for that DCC malarky. The speaker is screwed to a metal block. I'm told it sound even better if you turn it upside down.
The bogies are identical apart from the motor in one. I'd guess putting a second power unit in would be a doddle if you can solder wires together.
Of course, it doesn't look like this now.