Some people are a bunch of "D"s on this forum..... I don't know why everyone cops an attitude every time someone new comes on this forum.
What you're looking for is a loudspeaker. Like a PA type loudspeaker.
They also make Marine speakers that can be used outdoors. I saw a car at a show one time that had these types of speakers mounted behind the front grill.
You can also look up Outdoor speakers and see if you can find one small enough to mount somewhere safe, away from rocks and road debris if possible.
Getting a good quality set of loud door speakers can be the easiest way to solve the problem. Like the guys above were saying, just swing the doors open.
I've seen vids online with guys playing their speakers over 100 feet away sounding nice and loud. //content.invisioncic.com/y282845/emoticons/smile.gif.1ebc41e1811405b213edfc4622c41e27.gif
Good luck!