Switching Audio Output on iMac

Alright here's the deal-- I have a y-cable running from the headphone jack on my iMac, to my audio receiver. Half of the time, I like having the sound from my computer running to my audio system so I can listen to my music real loud. But, the other half of the time I don't want the sound running through my system and rocking my room. Right now, I just pop the audio cable in and out of the headphone jack when I want to change where the sound goes, but is there a way I can leave the cable plugged in at all times and just switch the output source from my comp? I've been searching and so far it doesn't seem like I can If you know how I'll, uh, be really really psyched.;D

Edit: REALLY psyched.

nothin?

I'd imagine that if you leave it plugged in then the iMac will always choose to send to the audio out port rather than the built in speakers. The only way I can see to do it is pull the cable out I'm afraid.

yeah that's definitely the only way without buying additional hardware.. which i'm searching for right now.. really stupid. Thanks Jobbs.