AHA! I fixed it :D :D :D
function gc_Hum17Vehicle::onadd(%this,%obj)
{
	Hum17SpeedCheck(%this, %obj);	
		parent::onadd(%this,%obj);
	%obj.playThread(0,"spin");
	Hum17SpeedCheck(%this, %obj);
	%obj.setWheelTire(0, Hum17tire);
	%obj.setWheelTire(1, Hum17tire);
	%obj.setWheelTire(2, Hum17tire);
	%obj.setWheelTire(3, Hum17tire);
}
Fixed! :D But now I have a different "problem":
function Hum17SpeedCheck(%this, %obj)
{
	if(!isObject(%obj))
		return;
	%vehicle = %obj;
%slot = 0; //Play the sound in the driver's spot
	%speed = vectorLen(%obj.getVelocity());
	if(%speed < 0)
	    %speed = 0;
	if(%speed < 10)
	    %vehicle.playAudio(%slot, Hum17idleSound);
	else if(%speed < 30)
	    %vehicle.playAudio(%slot, Hum17throttle1Sound);
	    
	schedule(500,0,"Hum17SpeedCheck",%this,%obj);
}
How do I make it play NO sound at all when it's not moving? What values and or inequalities do I put in?