First and foremost, extract your V4 folder. If it's a folder, the game can pick out that flare file (and many others) for every truck. \
Second, it has to be a track with a truck shop spawner to have multiple trucks at the same time.
Third, V4 trucks take up less memory than some of the trucks now a days with more details, so that's why you can spawn multiple V4 trucks.
Fourth, Sounds aren't that important IMO, so that's why I don't have an answer for you on your sounds question because I've gotten used to having no motor sounds on my trucks.
Hope this helps you out somewhat.