//First, declare your sound :
sound engine_wav = <engine.wav>;
// Then, use ent_playloop to play your sound : ( I use this code in my NewtonVehicle Function, dont know if is the right place, but it works.
var engine_handle;
engine_handle = ent_playloop (my, engine_wav, 50);
// Finally, modulate your engine sound with your engine rpm :
snd_tune (Engine_handle, 50,50,0 );
// To make the sound disappear, you can simple troggle its volume to 0 ( Zero ), the first value : snd_tune(Engine_Handle,vol, freq, bal)
Good luck.