That does not sound normal unless they are very cheap, old unpowered speakers designed to work with a computer that didn't have any internal speaker.
Most likely there is something wrong in your hooking up. Make sure the speakers are plugged into power if they have a plug. Also, the iMac has two 3.5mm jacks on the back. One is headphone and the other is sound in, make sure you have the speakers connected to the headphone jack.