Why do O Ihear imac startup tone when speakers off?

I have a pair of powered external speakers connected through the headphone jack.When they are not turned on I get no sound from imac speakers yet when they are off and i start up my imac i hear the start up tone.Would love it if my imac speaker worked for other computer sounds when my external speakers are turned off.Only use them for music/dvd playback.Any setting exist to accomplish this and keep my external speakers plugged into headphone jack?
Thanks

Don't think there is beside unplugging them.

The startup sound is pretty early in the OS boot process and will always use the inbuilt speaker unless you physically disconnect it. Once the OS is up, it can better manage the sound output jacks and use any external speakers that might be plugged in..

Regards

Thanks