I don't know of any right off, but it wouldn't be hard to make at all. You could consider a front audio pannle, as well. Check to see if you have the pins for it on your soundcard(just about all recent motherboards do, and most soundcards) These typically will mute your primary speakers when something's plugged into the front jack.
Or if you want an actual splitter(both going at once) radioshack carries stuff like that.