Unfortunately, you cannot select +Internal speakers+ in System Preferences
Sound pane
Output tab, if you have something plugged into the audio output port.
If you got an AirPort Express, its AirTunes feature would certainly work for iTunes output. However, I believe AirTunes would only play the output of iTunes. So it would work great as long as you were only playing songs and videos using iTunes, because all other sounds will come from your computer speakers. No need to change any setting manually, after you are done listening over the stereo system. However, it would not work so great, if you wanted the audio output of some other programs to be played from the stereo system; but there seems to be a third-party solution for that issue
http://www.rogueamoeba.com/airfoil/mac/
If you have separate powered computer speakers that connect using a standard mini-jack, one alternative is to get a USB (or possibly FireWire) sound output device (if you want to be able to change the sound output setting manually). The least expensive one may be the Griffin iMic.
http://www.griffintechnology.com/products/imic/
It provides a second sound output port over USB, so it should show up as a separate choice in System Preferences
Sound pane
Output tab. Therefore, you should be able to use the
Sound preference pane to manually switch the sound output from you stereo system to the computer speakers connected over the USB iMic.