I tested this in both the Eden Editor and on my own Dedicated server hosted on a separate machine.
Editor:
1. Place any AI operated vehicle down and give it waypoints.
2. Place an entity for you to control somewhere along the AIs path
3. play mission and hear engine sounds no problem
Dedicated Server:
1. Load mission with AI driven vehicles
2. Vehicles will have no engine sounds with AI operating them.
3. (optional) Kill the driver of an AI operated vehicle that HAS it's engine running and see how as soon as the AI is dead, the engine sounds begin to play as normal.