The startup sound volume is dependant on the iMac internal speaker volume. So in your case you might have set the internal speaker volume to zero (not muted) and then plugged external speakers / headphones to your mac. Now with your headphones / speakers plugged in, the volume control only applies to those.
You should un-plug your stuff and verify that the volume is not set to zero. This is all based upon the assumption that you have something plugged in of course.