Hey Mike, if you don't find the sound cars you're looking for, there might be another option with a little bit of work - I think there are several under-baseboard sound systems made for smaller scale layouts which have a variety of selectable "background sounds" including livestock noises - might be worth investigating these, I'm sure that with a little ingenuity and a battery power supply one could be converted to fit in a regular G-scale stock car?
I think that Noch make them, among others?
Jon.