You can simply use the built-in audio. It is a 3.5mm TRS jack. That would work but I don't like to do that. I'd get something like the PreSonus "audio box". It's about $150 and has pro-audio type balanced outputs.
There are other audio interfaces that are as good at the same price. Just look to see what outputs they have. m-audio, motu and lexicon and others are OK. Putput to a pa is not a hard problem so all of those do fine.
Just to add onto and clarify what ChrisA said, the built-in audio output is a STEREO 1/8" TRS jack. TRS can also be used for balanced connectors, so don't get those confused. Plugging the 1/8" TRS stereo signal straight into a TRS balanced line input on an audio mixer can result in very wonky sound (the stereo signals combine to cause serious comb filtering).
If you're going out of the built-in audio output into an audio mixer, you'll need a stereo-mono split Y-cable, such as this:
Typically you can get better audio quality and more varied/solid connection options with an external audio interface like ChrisA suggested.
If you're trying to get sound into a home theatre sound system, you could also use a Y cable, only with RCA instead of 1/4" connectors. Also, you could use the Mac's digital audio output with a Toslink cable into the optical input on your receiver.
Are you just curious as to what people are using or looking for a suggestions. I am using an Avid MBox 3 that I am very happy with. If you're looking for a recommendation, what is your budget?
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!