Well, I went to RadioShack and got the coupler. It turns out I already have a splitter in my house, but it's being used for something else, so I hijacked it temporarily to test this setup out, and it works great! The PS3 sounds much better on my computer speakers than it did on my crappy TV speakers.
So, I just ordered the Monoprice splitter (which is not out of stock), and it probably cost me much less than anything RadioShack might have had. So for now I'll keep hijacking this other splitter until the Monoprice ones gets to me soon.