Yes, you can hook up external speakers through the headphone jack, but you also need to use an amplifier, because the volume will be very low and plain speakers have too low of impedance to hook up directly to the headphone jack.
What I use is a small cable that goes from a 3.5mm stereo headphone jack to dual RCA jacks that I can plug into the auxiliary inputs of my home stereo (I got the cable at Radio Shack for $6) so I can amplify the music from iTunes.
I have an older iBook that I use just for music, but this will work for a MacBook, too, or any Mac that has an audio output, which is most of them. Adjust the volume in iTunes fairly high and then adjust the volume on your stereo.
You can also get powered speakers that plug into the headphone jack and have a built-in amplifier. They have an AC adapter that requires wall line voltage, but they’re handy if you don’t want to use your stereo amplifier. Check out these audio Speakers at the Apple Store or at amazon.com.
Amazon also sells portable speakers that use USB power from your MacBook and don’t require extra power.
Another possibility is to get an Airport Express which will enable you to send your iTunes music wirelessly to a stereo in your house. It’s smaller than a paperback book and doesn’t require an Internet connection; all it needs is to plug into a wall socket and then you can run an audio cable from it to your stereo. Take a look at the Apple AirPort Express, which will also give you a USB port to make a printer wireless, too. It’s easy to set up an Airport Express and if needed, have it work with your existing wireless network.