Certain iMac computers using Mac OS X 10.0 may not produce any sound through external speakers plugged in to the computer's sound output port.
Symptom
No sound comes from external speakers connected to an iMac.
Solution
Plug the external speakers in to one of the headphone jacks on the front of the computer instead of the sound output port on the side of the computer.
This issue may be avoided by updating to Mac OS X 10.1.