Hi Simon
Have I got Moos for you......
I have an LGB stock car with a cow sound board and it sounds pretty good. It repeats from a small selection of digital 'moo' sounds.
I have managed to get hold of three of the QSI 'Liveaction' digital sound cards. These were discontinued about '99 or so, sound very good and as Gareth (3valve) said, they have a pendulum which makes more contact as the car moves and causes extra 'agitated' sounds.
They are powered by a 9V battery and were for 'O' scale cars.but They work just as well in our scale. I am going through the process of making track powered circuits to power them.
They often come up on the US ebay and as they are fairly small and light, they ship for low cost over to us. The sound chip is replaceable and the other sound chips (horse, funfair, circus, braking glass, explosions etc etc) are still quite easy to find.
Here is the link to recently completed sales for the cards on US ebay. Even if the seller does not say that they will ship to UK, just send them a message and usually they will..ask for USPS international first class for the cheapest shipping for small items or for USPS International Priority if the items are a bigger and heavier.
I recommend them. They moo with many different moos and also the 'agitated' cow sounds are a lot of fun and very realistic!
Good luck if you choose to track one down, it will be worth it.