Just use a variable to check whether the mouse is down:
import flash.events.Event;
import flash.events.MouseEvent;
var mouseDown:Boolean = false;
var speed:Number = 0;
stage.addEventListener(MouseEvent.MOUSE_DOWN, mD);
stage.addEventListener(MouseEvent.MOUSE_UP, mU);
addEventListener(Event.ENTER_FRAME, oEF);
function mD(e:MouseEvent):void{
mouseDown = true;
};
function mUf(e:MouseEvent):void{
mouseDown = false;
};
function oEF(e:Event):void{
if(mouseDown){
speed += .2;
}
else{
speed -= .2;
};
};