Yeah, although you don't technically need anything to run sound out beyond the 3.5 mm cable, if possible I might suggest getting some kind of external audio interface, even a fairly cheap one, as it will not only improve your sound, but also because it will eliminate any hum or noise that I sometimes notice when connecting directly to larger PA or speaker systems. This is not always a problem, to be honest, but I notice it sometimes (large pa's are sometimes just more sensitive so they amplify hums that might not otherwise be noticeable).
If you're not having any trouble with that, then I wouldn't worry about it, but the other advantage is that most usb or firewire audio interfaces will improve the sound quality. Quite a bit, if you buy a fancy one! 🙂